Since chipping (GIAC), I occasionally get this noise on acceleration. I just notice it, it hangs around for a bit and before I know it, its gone...
http://au.youtube.com/watch?v=1_CSecpYrcA
5mins later, its gone:
http://au.youtube.com/watch?v=wO20hSR21cA
Power wasn't noticibly lacking, but...