Noise coming from passenger side bottom of motor

I have a 2003 Chevy Tahoe Z71, 5.3. 134000 miles when you step on the gas it makes a noise, I was told it may be the exhaust, but I had someone look at it and he said he wasn't sure but that it sonds like it is coming from the bottom of the engine. It doesn't make the noise when it is just runnig, but if you step on the gas it will make it, it's not a knocking noise. Any ideas? Thanks