I gave up on solving this issue after my third N800 exhibited the same behavior... My friends N800 is perfectly quiet. If I plug my headphones into his unit, no noise. I've learned to just accept it. At least when there is an active audio stream, the sound is clean. Nokia basically said it's normal (I called them about it). In general, I'm still quite happy with my N800. Rest assured, you don't have a defective unit. I just would't count on this issue getting resolved.

Frank