My son recently bought a 2006 Subaru Impreza with a manual transmission . He has driven it a few thousand miles & it seems to run fine with no obvious problems . Ever so often he hears a brief pssst . I haven’t heard it but from his description it sounds like a brief release of pressure . It does it at idle & also while driving down the road . As far as he can tell there is no rhyme or reason to it . Anyone have any idea what it might be ?
Either you or your son should join this forum.
Off hand, this sounds like the AC system, but check with these guys.
I was just about to say what @keith just said… sounds like the A/C compressor is cycling. Tell the young lad to make sure he doesn’t have the A/C button pushed to ON…or…if he has the Air duct dial on Defrost…I don’t think the A/C button illuminates when you are in defrost mode but the A/C compressor will cycle on and off to help clear the windshield. Sounds just like what you describe…
So…in essence…what @keith said… ha
Small exhaust leaks can make that sound. A failing exhaust manifold gasket for example. Usually it would be periodic though, in bursts. But the bursts can come and go intermittently.