Engine Light and hissing in engine bay

I've had what appears to be a common problem with my engine warning light being on. Dealer had a quick look at my last service a few weeks ago and reckoned it was the MAF. I cleaned it at the weekend with contact cleaner but it had no effect so im about to buy a new MAF.

I have noticed one thing however. There is (and has been for a long time) a hissing noise from somewhere directly behind the MAF (between MAF and wipers). I've inspected the few things in that area to try and find a leak but cannot. Thing is the garage said nothing about it when they did the service (they found every other fault they could try and charge me to fix:rolleyes:) and you cant fail to notice it, So i've assumed it was normal.

Does anyone else have this hiss or is this something that should not be happening. Its a 1999 2.0 SLXI (AQY)