Hissing behind dash

Hi all,

Got my car back (1991 Z16A GTO) after over a month away from it and noticed on the drive back a hissing/air sound coming from behind the dash seems to be there at idle but more noticeable under acceleration.

Tried to look through the forum and online and not found anything.

Is there any vacuum lines I should look out for or if you have any thoughts it would be appreciated.


is it freon leaking out? When I had a leak it sounded like a hissing sound behind the dash.

Unsure how I would tell, AC was off I believe it had done it prior to getting back so a total of 6 hours driving I’ve done since purchasing the car.

Had a feeling it may be a vacuum line but unsure what lines would produce a sound behind the dash can’t see any perished lines in the engine bay or no obvious signs of a leak in the engine bay at least

oh even if the ac is off you would hear a hissing sound from the freon leaking out. Does your ac work? But yeah I guess if it was doing it for 6 hours then its probably not that.

AC is still working as it should also

Are you sure that you are not hearing the rear turbo hissing if it is under acceleration?? It can be heard very good into the cabin if there is any hissing sound from it or the tubes around it.

It sounds like something behind the dash/ within the cabin, I will try and get a video and upload it may help diagnose.

I have the same problem on my both Dodge Stealth RT TT and I have no AC working for years. It is a very weird sound, it is a "strident " sound, only in accelaration or when the engine force a bit in hill per example. It appears after I change my timming belt on one car, and appears suddenly on my other one day.

I’m intermittently getting a similar noise but it seems related to the clutch pedal. I notice the sound changes and sometimes goes away when cycling the clutch. I don’t know if this is coincidence with another component or condition or its actually related to the clutch.

If you’re TT it might be your clutch booster leaking.

Clutch booster has vacuum assist?

Yes. It works similarly to a brake booster.


If you have upper vacuum line connected in reverse it will hiss every time you gather boost. This hose is S shape and can be installed in reverse no problem. It will also come with really hard clutch and break because assist is not working. And yes - 3000gt turbo has break AND clutch booster. Just look at your legs - you can see smaller but similar to brake booster in there.

I had similar problem on my Galant VR-4, it was valve on vacuum pipe to brake booster.


In my case I think it was one of the vac lines, noise went away after doing the vac line delete and finding a few hoses going nowhere near the back of firewall