Revs increase 1000-1500 when clutch depressed

[aye you]

Hey all,

on the drive to work today i noticed an odd noise from under the hood. A mix between air hissing from a join and a grinding type noise. Anyway, at idle to car used to hold 600rpm, but now its jumping up to 1200ish, when driving if I take my foot off the accelerator it'll hold the rpm (crusie control off) and when I clutch in the revs will either stay where they are until I clutch out in the next gear, or will jump from 100-1500rpm and hold again until I clutch out.

At a complete stop the rpms drop back down to idle.

Ideas? Vaccum leak? air leak after MAF, i'm going to clean the ISC tomorrow morning. the only change i've done in the last fortnight is topping up the power steering pump (which i may have overfilled slightly)