Hissing from passenger side under the dash

[jasonpriddle]

Hello

I have search for the answer to my problem, but found nothing.

My problem is a hissing coming from under the dash on the passenger side. The sound is during acceleration, but is not consistent. The same noise is made during the operation the the heating/cooling system. Such as when I turn it on or change the settings.
It seems to me like a leak in the air supply system, but I don't know enough about it to diagnose it.

It is a 2008 sport trac limited. V8

I have a pic but can't seem to upload it.

Any help would be appreciated.
Thanks

 

[jasonpriddle]

The noise is coming from this part.

 

[arco777]

The vacuum reservoir holds extra vacuum to help drive vacuum-operated accessories when engine vacuum is low (such as high RPM operation or if the engine stalls or is off). It needs to be airtight and so should the tubing going to it. If there is a hole or crack in it, either patch it up or replace it. RTV works pretty well to fix vacuum leaks.

Some hissing noise is normal as the system uses vacuum to change HVAC settings, but if it is leaking you'll want to fix that before it gets worse.