Search didn't help. When I turn off my '91 240 it makes a SHHHhhh sound from under the dash I think. It's not a hiss exactly more of a gentle air excaping sound dropping off over a couple of seconds. The car goes well and it has done it for years, but I still don't know what it is. Any ideas?

 

The sound you hear most likely is from the vacum tank that lies within the center console. It holds vacum in the tank in order to open/close air vents in the dash. There is a vacum line that runs out of this tank through the firewall to the intake manifold. Right at the intake manifold, there is a check valve that is black & white. What has happened most likely is that this check valve has failed. Pull it out and see if you can blow through it from both ends. If you can, then it is bad and should be replaced.

Same thing happened to mine and I had the exact same symptoms

 

Thank you.