I have a rattle under the hood that sounds like it is coming from the pulleys of the drive belt. It's only noticeable when the car is sitting idle or stopped. When driving it goes away. The rattle noise sounds like the noise you hear when a diesel engine is running but a little louder. Please help!
You may have an idler pulley that has a bad bearing, this is the pulley that the belt rides on. Check your oil level to make sure it is at the full mark and you do not have a motor knock due to low oil.
Your harmonic balancer may also be causing this issue, which also is a larger pulley on the lower end. They can age and cause severe noise at idle and the knocking sounds can plane out as you gain RPM*s.