I get the same noise on my '88 190E 2.6 for a couple of minutes on cold mornings, but it doesn't seem to affect HVAC operation, so I haven't dug into it.

Also, I hear a whistling noise, again when cold in the morning that I think is an aspiration blower to detect passenger compartment air temperature. Again since HVAC system operation seems normal I haven't dug into that one either.

The Mercs of that vintage are very well engineered and built, but most everything seems overly complicated. Consider the M103 throttle linkage, and I dread the day that the HVAC system needs more than an evacuation and recharge.

Duke