I have a 1990 TC that has, since the day I got it, had a noticeable, yet low volume whistle, at all speeds. The car only had 27k two years ago, and now has 50k. The car sat in a garage for 8 or 9 years before that, unused. I do not think this a TC issue, specifically.
When I am cruising or accelerating, I hear this sound. The frequency goes up the harder I press the accelerator. When I go down hill or coast, it goes away.
As a footnote, until I replaced the AC condenser late last summer, and pumped down and refilled my AC, I had full control of my air outlets - except under acceleration, when the air would all go to the defroster and cooling went away. When I bought the car it did not have functional AC. I discovered the system had totally leaked out all of the Freon. So I refilled it with r134 and leak-stop. Before and after the condenser replacement, I actually had good AC (before the replacement, though, I had to make monthly leak stop and charger additions, to keep the cabin cool).
To complicate issues, I only had full AC for about a week after the new condenser installation. I checked all connections, and put gauges on it, and all seemed normal, except no AC. Suddenly I had no AC and no control over the air vents, air always coming out the defrost vents. And even with the temp slider to the cool side, I got heat.
Seems suspiciously like a vacuum leak, right? So I have replaced many, in fact, most of the hoses I could see and find under the hood to no avail. I have not removed the dash as of yet.
Any thoughts? Is the AC loss a vacuum leak, which could explain the whistle, and the loss of air vent control? Or is it something else altogether, and the AC issue, the whistle, and loss of vent control, unrelated?

It definitely sounds like a vacuum leak.

I had the same issue in my Jeep with the AC. For me, it sounded like a super charger whine.

The defrost setting is the default when the actuator system goes down so your windows stay clear (safety thing). It also took out my cruise control and my 4WD. But each system went down independently as the vacuum leak got worse (4wd went first, then A/C then finally Cruise).

Replaced a few vacuum T-connectors and all is well again.

I think you definately have a vacuum leak. On another note, The oil that lubricates the AC system (compressor) R-12, is not compatable with R-134..

i replaced the Receiver Drier / Accumulator, and the condenser. and then i pumped the system down. as part of the change-over to r134, i added new oil. do you think any r12 oil could have remained?
thanks for the info, everyone!

The vacuum hose/leak you are looking for is easy to find. Just stick your head in the drivers side footwell, and look up for the yellow vacuum line. It plugs onto the end of that vacuum 'can'that has a barb on it Easy no?

Alan

Sorry, but I'm afraid I do think there is still old oil in your system. The normal procedure is to pull the Compressor Drain it and refill, Then Run a flush thru the system before pulling it down for 1/2 hour and refilling everything. Good Luck, Lou

cr@p!
thanks lou. i guess i'll need to check this out

alan,
thanks for the suggestion. i found the yellow hose just where you said, although it took me 5 minutes to get back up ;-). started to think about 911...
anyway, the hose was on. so with the engine running, i checked things a bit and pulled the hose off to see if there was vacuum. nothin'! i tried it with the fan on and off, the a/c on and off, and pressed the various vent control buttons to see if that changed anything: still no sign of air being sucked through the hose or canister. seems fishy. at least i still hear the compressor turning on and off, so i feel confident it hasn't failed.
pulling the entire dash apart maybe next.
maybe the vent and a/c issues are unrelated to the whistle which has been going on much longer.
any thoughts about that?
fyi - after a few hours at a dealer, it was suggested they "might" fix it if they just start replacing stuff. sounds... scientific AND professional. or like what i have been doing the last many months.

That Yellow hose should be the primary feed. directly from the engine. follow it back, there is a very good chance that somewhere along that hose tothe engine is the problem.