Why is my hot water heater making funny noises?

Recently, my hot water heater has started making gurgling/clanking noises whenever the burner is on. The heater is in the basement right below my bedroom, and we can hear it from bed.

Two things happened on the day it started making noises:
1) We turned the temperature up from “vacation mode” for the first time (we live in Tucson. when the normal “cold” tap water is over 90 degrees, you don’t really need a hot water heater).
2) We drained the sediment from the bottom of the tank, following the instructions. Apparently this is supposed to be done every 6 months, but we really doubt that whoever was living here before ever did it. We drained about 2 gallons of water from the tank.

I personally don’t think that the sediment has anything to do with it, although the noise does sound sort of like gurgling of air in the tank, as if it’s not all the way full. However, the tank should’ve refilled all the way, just as it would have if we’d turned on any tap in the house.

My boyfriend thinks the sound is sediment banging around, but I think that sediment is more like sand than big rocks, and how can sand make that much noise? And why would the water get agitated enough to throw sediment around?