My car makes a rattling noise under the hood when the car is in motion. I can be pressing the gas and it makes the noise, or coasting in neutral and it makes the noise. Sometimes it is very* loud, and sometimes unnoticeable. The noise stops when I am idle at a light or parked. Unfortunately since I can't pinpoint the source under the hood (when idle), I have to guess that it is coming from the under the hood in the center, but slightly skewed to the right. Any ideas?
Typically these noises are exhaust related. If you cannot find any loose exhaust heat shields or worn components, I would check to see if the alternator is buzzing around. The alternator mounting bolts sometimes become loose over-time. Under extreme circumstances, you will have to replace the alternator bracket if the threads are badly worn.