HDD Power down?
I used m0n0wall for a while and I loved the silent feature to power down the hdd after several minutes of inactivity.
can pfsense run only in ram?
If you want a completely silent system you want a compact flash card, not a hard drive.
Having the hard drive power down would only cause you a delay when it was needed and had to start back up. Hard drives really arn't loud these days. I ran an old 233 dell box with hard drive and it was barely audible unless up close.
However if you want it dead silent get an IDE to compact flash adapter and a compact flash card. I have this setup in my nokia ip330 box, costs around 30 bucks for the card and adapter. Works great.