Sounds like a vacuum leak in the area of your temperature/heating control system.

Try to isolate the leak. It could be that you have more than one leak: One at the transmission or the line going to it and one at the back of the control unit in the dash.

Try listening down near the dash with the engine running, even take the asktray out. The hissing is telltale.

Keep us posted,

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)