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

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!