Ford Focus 2002 1.4 idle problems

Hello guys, new to this so please bare with me, erm Ive got a Ford Focus 1.4 2002 with 25;000miles on the clock and recently( around a month ago) I was stationary at some traffic lights and noticed that when it's parked up the engine sounds likes it's going to stall the revs start bouncing up and down from about 800-1000 and  there's a hissing noise coming from the engine when you open the bonnet. so I took it to my local garage where I get my MOTS done and he had it for a few days and changed the "idle control valve unit , throttle body , co2 sensors, " with second hand ones and he can't find the fault he's ran emissions tests,disconnected the battery after changing those above. He's checked all the pipes in the engine apparently. I've been looking online at all these posts and seen many topics about this , has anybody had this problem or solved it before 

thanks