Replace the fan. I bought a 3.4g P4 a while back. It was pretty top of the range at the time (obviously things have moved on since) but the fan noise was nasty. Fan was strictly linked to temp/workload and when doing anything intensive it was hard to hear anything over the noise from the fan.
The solution was simple - I shopped around for a half decent fan. For around £15 ($30) I got something so quiet I can't realy hear it and the temp is lower than before. Well worth the little bit of cash.
And in my case there was no fault. It was simply (and badly) designed with a shit, noisy fan in it.
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