Just thought I'd post up for others that experience this. A few weeks ago, I started to hear a whirring noise under the hood that was engine rpm related only on my '02 WRX. Not load dependent; just rpm dependent. Strangely, it would somewhat come and go. With the hood open, it sounded like it could be coming from the alternator or the AC compressor. It turned out that the noise came from the idler pulley on the belt tensioner for the AC belt. It was a cheap, easy fix. I bought a new Dayco 89029 metal pulley w/ bearing from Advance Autoparts and it seems much more substantial than the plastic OEM one. Problem solved.