These can be tricky to diagnose with out actually hearing the noise, but from what you mention I feel that it may be over head gear and engine oil pressure, I feel it is time to talk to your local mechanic and let them have a listen to this one.
March, 10, 2013 AT 5:47 PM
Check the Alternator belt Tensioner for excessive rattle at idle... have someone rev the engine while you look at the tensioner.
If the tensioner roller moves LESS above idle speeds, then you have a one-way-clutch failure of the alternator.
If you can get to the alternator, you can check the clutch while it is still in the car: (this clip also shows the tensioner vibration / rattle)