danrun

I've read some things on this but nothing really matches mine. I have a 99 t/a with 79k miles. The more I get on the gas the louder it is, doesn't matter about rpm's. And it only does it when I'm on the pedal. It doesn't when im coming to a stop. And what is so weird to me is that some days it will do it and someways it's as quiet as a mouse. Oh and it's the stock 10 bolt with stock gears to the best of my knowledge. Any ideas?