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reverie's avatar

Car mechanics: Why has my car developed this metallic rattling noise?

Asked by reverie (1777points) June 24th, 2010

About a week or two ago I noticed my car had started making an unusual sound, which is audible when the windows are closed or open. The sound appears to be some sort of metallic vibration rattling noise coming from somewhere under the front bonnet of my car, and it tends to sound when the car is idling or travelling at low speeds (approximately 20mph or less). Whilst the sound is fairly loud and clearly audible, it isn’t so loud that it’s alarming or impossible to ignore or talk over. It happens no matter what gear the car is in. Whilst the noise tends to happen when I accelerate (presumably because of the increased vibration), it also happens when I am not accelerating and just coasting at a low speed. The noise is not constant, and only seems to happen during periods of increased vibration in the car when it is traveling slowly. The sound stops entirely when the car is traveling at speed (25–30mph or more). I have not noticed the noise affecting the performance or handling of the car in any way, and I have been driving it with the noise for a couple of weeks.

Yesterday I opened the bonnet and turned on the engine, in an attempt to isolate the source of the sound. Whilst I couldn’t see anything visibly loose, it seems to be coming from an area at the front of the engine (if you are standing at the front of the car and open the bonnet, the noise comes from the side of the engine nearest you, more towards the left hand side).

In case this is relevant, here is some additional information about the car:
– The car is a 1999 (V-reg for those in the UK) Nissan Micra with manual transmission, approximately 80,000 miles on the clock.
– Its last service and MOT was last October and no problems were reported.
– When examining under the bonnet, I noticed that a screw was missing from the top of the car engine, but on the opposite side from where the noise was coming from (top right). The inside of the screw hole is clean so this may have come out recently, but I have no way at all of verifying this – it may have never been there or may have been lost ages ago. I don’t know whether this is just a coincidence, or something associated with the noise.

I am concerned about the noise, and therefore think I am going to need to take my car to be checked at the garage. I would like to ask on Fluther about this first, as advice about what this noise might be would be incredibly helpful, so I can be a little more clued-up when I speak to the mechanic. I guess I’d also like to know whether you think this might be a really serious problem that needs attending to immediately, or whether I can risk it and wait a while before going to the garage (I am really short of cash at the moment!).

Sorry for the long post, any help or advice you could give me would be tremendously appreciated!

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27 Answers

john65pennington's avatar

Have you checked the engine oil? if the oil level is low, the valves will rattle. this is a metallic noise. john

reverie's avatar

Hi there John, thank you very much for your suggestion. I hadn’t actually looked at the oil level, no – I will take a look when I get home tonight! Thank you so much for your help, once I’ve had a look I will post an update here with what I see.

Is oil level something that gradually gets lower when cars are functioning normally? Or, if it’s getting low, could it be indicative of a leak or some other problem?

Tropical_Willie's avatar

@reverie Don’t drive it without checking the oil. The other possibility is the fan clutch will sometimes make a knocking or metallic sound.

reverie's avatar

@Tropical_Willie Thank you very much for your advice. I’m currently at my workplace and I drove here this morning, and it’s about a 16 mile drive back home. If I check the oil here and find it is low, is it safe for me to drive it home to refill with oil there, or should I head to a garage to refill the oil as soon as possible before doing the full journey?

Tropical_Willie's avatar

There should be two marks on the dip stick. The lower of the two is usually one quart or liter down. If the drip stick is dry DO NOT drive it.

reverie's avatar

Thank you so much for your advice. I’ve just been out into the car park to have a look at oil levels, and the oil is about half to three-quarters of the way full (i.e., between the “min” and “max” marks on the dipstick, the oil is about 50–75% of the way towards the “max” mark, so it doesn’t look like it’s low on oil). I’m really not sure what to do with it now if the oil doesn’t appear to be low… I guess I should take it to get looked at asap!

arpinum's avatar

Check heat shields and their fasteners. Sounds like some rusting heat shield could be causing it to droop on the exhaust system. The rattling would be it bouncing off the pipes.

CMaz's avatar

Are you sure the sound is coming from where you think it is coming from?

The catalytic converter will do that.

reverie's avatar

@arpinum Thanks very much for your suggestion! I don’t really know very much about cars at all unfortunately, so what you mention is probably something I would need to get checked by a mechanic!

@ChazMaz Thank you for your suggestion. Whilst I couldn’t be confident of the exact area, I’m pretty sure that the sound is coming from the front of the car. With the car switched off, I opened the bonnet and had my boyfriend look inside, and then I got in and switched the car on (it usually makes the rattle as the engine starts too) whilst he watched and listened to the whole process, so he could get a really good view and listen to where the noise seemed to be coming from. We did this a couple of times and he seemed to think it was coming from this area on or near the engine, but obviously we can never be totally certain, although I’m pretty sure it’s not coming from the back. I’m worried, really hope it isn’t something expensive!

CMaz's avatar

If you don’t already know. The catalytic converter is part of the exaust system. IE back of car.

IchtheosaurusRex's avatar

Just for laughs, look underneath the front of the car to see if the chin fairing has come loose. This is a plastic lip designed to deflect wind. They’re easily damaged if you run into a parking block. They rattle in the wind, but it tends to stop at higher speeds.

Coloma's avatar

I had the same noise only coming from the back end of my car awhile back, rattling at low speed and going over bumps or potholes..was the rear airshocks…maybe an alignment issue or struts. ?

Scooby's avatar

Could be the heat sheild off the manifold! :-/

jerv's avatar

With the vehicles I have owned/driven over the years, I have heard a wide variety of weird noises. Rattling noises I have had over the years have been caused by:

- Loose body panels
– Loose dashboard panels
– Loose heat shields
– Loose air cleaner housing
– Misadjusted valves (unlikely in your case since most modern cars have hydraulically-adjusted lifters)
– Worn suspension components
– Random crap in my glove box, center console, or door pockets
– The ashtray

It’s hard to say without actually hearing the sound.

Coloma's avatar


And you know that NO BODY ever hears the sound when you need them to!

My mechanic alway’s says it’s the loose nut behind the wheel. lol

Scooby's avatar

I’d check it for free if you’re North east! ;-)

jerv's avatar

One other source I forgot to mention is spark knock which is why I can not run 87 octane in my ‘85 Corolla. My ‘87 Corolla required 93 octane for similar reasons. The sound going up a hill, accelerating, or at low speeds where the engine was at an RPM below it’s powerband, the sound could be compared with a soda can with a few BBs being shaken.

It is highly unlikely that that would happen in a modern car (like yours) without tripping the Check Engine light unless the CEL bulb is out since the engine computer has sensors and programming to deal with that, but I had to mention it for the sake of completeness.

@Coloma I have yet to see that. I’ve seen a loose nut behind the wheel cause sudden acceleration or braking, weaving across four lanes of traffic, and just generally being an ass-hat though.

Coloma's avatar


I was kidding in loose nuts behind the wheel when diagnostics fail to pinpoint the noise. Never seem the serious type…lol

jerv's avatar

@Coloma I just have a dry humor. I suppose that a driver distracted by a cellphone or some soccer mom filled with road rage don’t qualify as “a loose nut behind the wheel” then?

reverie's avatar

Wow! This is all incredibly helpful – thank you so much everyone for all of your suggestions, I really appreciate it. I’m so impressed with all of the knowledge here.

@jerv – you’re quite right, I wish there was some easy way of recording the sound, as I think that would help a lot! Actually, the sound seems to have reduced a little, but it’s still audible. It’s definitely only happening intermittently when it’s idling, at a low speed in a low gear (i.e., below 20mph in 1st, 2nd or 3rd gear), and at slightly higher speeds in inappropriately high gears (i.e., slowing down to 30mph in 5th gear, it makes the noise).

@ChazMaz – haha, with my poor knowledge of cars, I did at least know that :P

Well, I’m trying to get hold of my garage at the minute and hopefully someone will be able to look at it soon. I’ve had to drive it again today (not really avoidable) but will avoid using it again until I know what the problem is – I’ll let you know what the mechanic says, there have been so many great suggestions, I bet someone here is spot on!

reverie's avatar

@arpinum @Scooby @jerv You were all right with your suggestion! I’ve just had my car back from the garage, and it was a loose heat shield rattling. The mechanic welded it back in place for me and it cost next to nothing. Yay!

Scooby's avatar


Excellent!! ;-)
Rattle free driving, there’s nothing quite like it, glad to help out………….. :-)

Tropical_Willie's avatar

Good news—Keep an eye on it. The next time it makes noise you’ll now know where to get it fixed.

brettjones's avatar

Sorry to say think it is gearbox related.

Renault_no_no's avatar

I don’t know if this thread is still live but after stripping most of my fuel and starting systems for a rebuild after a complete engine failure, I have developed a similar rattling in the lower portion of the transmission. My vehicle is a Renault 1.9 DCI Grand Scenic II and an absolute nightmare for the home mechanic to maintain. The sound is a ‘tinny’ rattling that I sincerely hope is not the presence of a fallen centring dowel from the starter motor that has become lodged at the base of the bellhousing. Does the learned community believe that this could be a possibility without me having to remove the transmission facing to retrieve something that may or may not exist on a Renault starter motor? Haynes mentions the dowel and warns to not lose it but other starter motors seem to be absent this apparently vital piece of metal and I’m wondering if it’s something only on the petrol models?
Aside from this, I have been working under the vehicle and agree that it could be the razor sharp heatshield vibrating against part of the exhaust system. My other theory, as I have renewed the ABS modulator and all brakepipe is the possibility of loose clipped pipes vibrating against the vacuum-servo unit or against the transmission housing.
I’ll keep this thread posted as to my findings and may be inclined to post some helpful topics on how to remove the starter from these diesel models from above the vehicle without following the annoyingly concise Haynes manual that requires (in extremities) cutting the exhaust to remove the catalytic converter unit and removing unnecessary wiring and pipe, especially if your nuts and bolts are a seized and corroded as mine!

jerv's avatar

Old threads never die here :)

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