I just purchased a K&N CAI (see my thread) and it produces a very annoying humming sound at idle. The sound it produces at higher RPM's though, is nice! It came with great installation instructions with pictures for every step, the hardest part (if you can call it that) was finding the 2nd screw that held the lower "air box" that is down by the fender. I had trouble getting access to that screw without taking the fender off. I eventually did it, but I had to twist my arm all weird to get to it