im suffering with the exact same problem mate, when my bike is at idle and the throttle is given a quick twist it also makes the noise its like theres a stone in the headers or sumin its a weird noise but iv had the headers off and nothing. what kind of pipes do you have on, mine are stainless and iv heard they can rattle for some reason, mine is with a mechanic at the moment and hes trying to sort it. i will mention tho mine is a cbr400

well just got my bike back and the culprate has been found this lovely chunk of metal was inside the headers, the mechanic said its a balancer and woulda been tacked in there and it came loose he had t cut the header and re-weld it back up to get it out, the bike is spot on now :)