Rattling Sound

I have a '99 Rodeo V6 4WD that has for the past month had a rattling sound. It seems to be coming from the right half of the hood or in front of the passenger side (when you are sitting in the drivers seat). At first it seemed like it would make this sound just when the car was cold and I was driving it. But now it seems to do it more often. I have mainly noticed it doing this when:

-I first get going and reach the 2000rpm-2700rpm area while pressing the gas (it never makes this noise when I am not giving it gas and the sound will immediately go away if I let off the gas)

After I reach 3rd or 4th gear and am around 2000rpm-2700rpm and the car is warmed up it will almost never make this noise, but it does sometimes. I never notice the noise at any other rpms, but I am often around the rpm's mentioned above.

If I do not have the stereo on it sounds really loud. Any ideas?

I have recently replaced the idler pulley and serpentine belt. Isuzu has recently replaced the intake manifold gasket, fuel sending unit, turn signal relay box, front R wheel speed sensor, rear rotors and pads, repacked/regreased front wheel bearings, replaced front pads, and they machined the front rotors. All of this was done before the noise began to occur.

Any help would be greatly appreciated. Thanks!

Sounds like the timing belt tensioner to me.

The timing belt tensioner? Is this an expensive fix at a shop?....or by myself? Thanks!

The part costs $70- $100. The labor is expensive because it takes some time to change it. I changed mine myself, but I have no idea of your mechanical ability. If you do have a bad t-belt tensioner, you should replace the t-belt while its apart and the water pump.

Mine does the same thing. There is a exhaust shield... Check to see if that is making the noise.. My dad stuck some towels up near it to put some pressure on it and it stopped making the noise..Obviously I removed the towels.. hah