Volvo Repair: vacuum leak, vacuum leak, engine vacuum

Question
89 240  I just purchased this car and while driving home, I began to hear a loud shrill noise from in the dash.  I assumed it to be a vacuum leak from the vent system.  Removing the the line from the right side of the white plastic vacuum canister located under the heater housing, makes the sound go away.  I assume this line is the engine vacuum.  I then removed and plugged the line on the left side.  With this line plugged and shutting the car off, you can hear vacuum escaping from somwhere.  Are there any other outlets on that vacuum canister other than the two on the ends?  Is there a check valve incorporated somewhere in that canister?  It is kind of hard to see under that heater housing.  How is that canister removed?  Thanks

Answer
Brad, first check to see if the hose is split at the vacuum valve in the engine bay.   That white canister never goes bad.  If the engine bay hose is okay then the diverter valves have a leak.  Those are not fun to remove.  Ther also may be a line that has come off behind the diverter valves.  Roger