Erratic Idle Revs

Car is a 2.0TFSI (BWA).

Yesterday after driving the car around 5 miles home I noticed it idling at around 1100 rpm when I got into the driveway. I kept it running a while and it would vary between 1000 and 1200 and then suddenly drop to 800 before rising again. When I stopped the car I could hear what sounded like a hissing noise (like a pressure release) that lasted a few seconds and then stopped. I've noticed since I've had the car it has always idled pretty high at around 1100 rpm when first started and then would settle but it had never done this before once warmed up.

The only other problems I have had are the usual coolant warning from cold that goes away that I presume is the bottle sensor (not getting any coolant loss) and a noise under the timing belt cover that lasts around 5 minutes before going quiet after starting from cold. It sounds something like maybe a tensioner bearing or something that's a bit lazy and then frees up (just to describe the noise). I'm bringing the car in to have both the above looked at once I can get time off of work as it's still under warranty and I can get the revs problem looked at too. But I'm just wondering if anyone's seen this before as I would like to plan for not having the car for days on end if it's something that's hugely wrong.
Thanks.

Started getting really down on power and seemed to be misfiring while accelerating hard.
Took an chance from some bits I'd read and got a new breather (PCV) rev. N, and fitted today. So far so good (touch wood).

As an aside to this, are there any mods out there to get rid of that absolutely insane engine cover setup, or is my only option to fit an induction kit? That's the first time I've had it off and most of my time was spent searching for the rubber mounts when they fell off.