help please with heat, vaccum element

[gidpor]

I'm hearing this sucking sound + a remote flap trying to do something.. the sound comes from the driver side center console area, when I turn the left temperature dial up to get heat. The blower fan does not seem to be involved in this issue one way or an other.
The strange thing is, This only happens with the left dial - . Could it be a failing vaccum element (or a short ) ?.Which of the several elements could be the culpirt ?
Has anyone encountered this problem before ?
thanks.
P.S.this is a 1987 300SE/Euro

[gidpor]

while driving the car this morning I realized I was wrong in pinpointing the source .
Actually it seems now that when turning either of the temperature dials, not just the left one - I create the problem mentioned above.

[engatwork]

I found the best thing to do is to test each one and replace the ones that do not hold vacuum.

[gidpor]

I was figuring out something in that direction... - would you know if the climate control vaccum actuators are accessible from the side panel(s), or do they require the dreaded "remove the dash" ?