2005 Pontiac Grand Prix Car making weird nose when idleing

I have a 2005 Pontiac Grand Prix with the 3800 V6 Supercharged engine that has a weird knocking noise. It doesn't sound like the engine is knocking though. The interesting thing is that is goes away instantly when you accelerate. If you even barely touch and hold the gas pedal the noise goes away. If you park the car, the noise usually goes away by the time you get the hood popped and can listen for it. If you are driving down the highway and let off the gas you can hear it but it goes away as soon as you accelerate again. Any ideas?