sounds like the front left wheel well, but my dad was with me and he said it sounded like the front right wheel well when he heard it. To me it sounds like someone shaking a tin can full of screws almost, not really a knock. I haven't noticed it in comparison with the rpm's. Just seems odd that it goes away when i take my foot off the gas. I was reading a current thread on someone's cat coming apart. The only thing i can think of with it going away when i stop accelerating is it has to be engine/exhaust related.