Blowtus

it's hard to tell, but I'm pretty sure my low rpm (below about 3000, it still seems to be there after that but blends in with the general whir) crappy noise has been slowly getting worse. It's very hard to describe, my only hope is someone else out there has had similar...

it's almost like some variety of valve is spitting out air... sounds like it's coming from the engine bay rather than the exhaust out the back. not a 'ppsssssht' noise, more of a 'phtphtphtphtpht' (in the same vein as the dakdakdak of an old vw beetle)
well there's my attempt... it's even worse than I expected, but have a think for me eh?

edit: it doesn't make the noise when I rev it in neutral, it does make the noise when I switch the car off...