ok here is the deal.

My car (81 GSL) will start and run for a few seconds then you hear a "thunk" (hollow sound kinda like a tennis ball cannon, but softer) then the car stalls.

Once the car stalls you can hear a hissing from near the drivers side of the engine bay but I cannot pin point the hissing, any ideas folks? cause this is pissing me off.

 

Thet thunk is probably a misfire after the car dies. For awhile my car would do taht same. I would shut it off and a few seconds later I hear that thunk sound, like you describe it.

As for the hissing sound...dunno really.

 

dunno the thunk sounds waaaaay too soft to be a backfire. I a standing looking into the engine bay as my wife starts it. It may be a tiny backfire but highly doubtful. It sounds like somebody dropped a wrapping paper tube...bout that loud

 

Well, it may be tiny and that's just my idea. Of course, I do have cats...old school headers to presilencers.

 

on closer inspection it may be the shutter valve snapping closed, then the car stalling????