UPDATE: I pulled the glove box along with the underdash panels and various shrouds and ducts, and was able to observe the motion of 3 visible HVAC actuator rods when the A/C is turned on. The whistling would start when one of the rods reached the end of its stroke. The culprit turned out to be the actuator pod connected to that particular rod. Removing it was one of the hardest things I have ever done - I think you're probably supposed to remove the dash when working on these things. It took me about 3 frustrating hours of contorting under and through the dash to locate and remove the faulty pod. Once removed, I turned the A/C on and the annoying whistling is gone.

Interestingly, the offending pod didn't seem to be moving its rod and corresponding lever through the full range of motion, probably due to the vacuum leak. So it seems that some function of the HVAC was not operating properly. But with the pod disconnected, I move the rod fully up and fully down, and can't tell any difference in vent output. Does anyone know what these 3 particular rod/lever/flap assemblies actually do? It almost seems like the defective pod isn't really essential. Perhaps it controlled the fresh air blend?

Sitting in my hand, the pod definitely has a leak and doesn't even hold vacuum very well. I took it apart to see if I could find a tear or crack somewhere in the diaphragm, but none could be found. It just looks like the parts no longer fit together well. The pod is made out of plastic, so I suppose some warpage is definitely possible.

By the way, I confirmed that it was the source of the loud whistle by encapsulating it in my hand with the A/C running, which resulted in the whistling being greatly muffled. Just thought I should point that out - when the whistling was occurring, due to the high pitch it was almost impossible to tell which direction it was coming from. At one point, I couldn't tell if it was coming from the pod or from the switchover box a few inches away. So muffling it helped me to be 100 percent certain.

Oh yeah, don't make the mistake of twisting the pod and disconnecting the rod from the lever at the top!!! It took me about 45 VERY frustrating minutes just to figure out how to reconnect it. I was practically in tears. I can't even imagine what I would have done if the lever broke!!!

Since I'm at work today, my wife is going to the dealer with the pod, in hopes of obtaining a replacement. Hopefully they sell these things or can order them. There was a blue sticker on it that appeared to have a barely discernable part number on it.

Once I have the new pod, I cringe at the thought of installing it. It will be painful and tedious. But I'm not doing this for free - my coworker will be paying me fairly, believe me... Can anyone tell me what a dealer or good indy would charge for something like this? It's not very technically difficult work, just very painful and time consuming. All in all, it will probably take me about 6 to 7 hours total. My friend will be thrilled - as great a car as it is, it was almost unbearable driving with that annoying whistling sound!

I'll report back to let everyone know how it goes...

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280