I have a 2006 Grand Prix with 50,000 miles. I just bought it 5 months ago and now there is this loud knocking/thumping sound when idle. When I start to accelerate the rhythm becomes faster but then disappears until I come back to an idling state. I have had this checked out 3 times with 3 different places and no one can tell me what is wrong with this thing. It doesn't seem to drive any different and no warning lights have ever come on.

Any thoughts?