the air isn't for the hard drives, I'd happly run hard drives without any air at all. The air is to feed the GPU and generally keep the case temps down. (it's the only intake avenue I have on my p280 case).
Ah a P280. I used 800RPM Slipstreams in that case, to feed a fanless HD6850. Worked great, and the hard-ish mounted (still not as good as suspension) Caviar Green drives were more noticeable by far. Photo
Hell, even after I rigged up a suspension mount, the Slipstreams @ 400RPM were still quieter.Photo
Without the intake fans, the GPU fans are always groaning because they're spinning up fast even without running games.
Could be due to youtube videos or movies triggering GPU acceleration and warming it up? Also, have a play with the fan speed curve using MSI Afterburner (or a similar program) if you haven't already.