Maybe this is old hat but I thought I'd share the way I quietened down my desktop's noisy case. It's mostly a quiet machine but under certain conditions something would resonate and produce a really annoying buzz.
This is nothing earth shattering or clever, I just stuck some insulating tape all around the contact area between the side panels and the case so that the first time I put the panels back on I had to force the little retention hooks/catches on the top/bottom edge of the panel through the tape. The difference it has made is really quite surprising considering it only cost me about 4' of tape so if your case buzzes I'd give it a try.