Yesterday I noticed the A/C on my 2000 TT wasn't cooling as much as usual.  When I drove it today, I noticed a fairly loud rhythmic noise in the engine compartment that went away when I turned off the A/C.

What might the problem be?  Will it be okay to use the A/C until I get the car to the dealer?  With temps in the mid-90s here I'd be pretty wilted without it, but I don't want to do any [more] damage.

Thanks.

Most likely culprit would be the compressor seals, compressor clutch, or the belt that turns the pulley. Since it stopped when you shut off the AC that leads one to believe the AC compressor or its clutch is acting up. AC Clutch engages electronically to cause the compressor to turn. Your compressor seals may be going south, which would explain why the AC is running tepid. Most every repair shop will tell you that you need to replace the compressor because the seals and the clutch are integral to the pulley and compressor. Bad seals will allow the refrigerant to leak out along with some oil. Eventually the clutch will not engage and the compressor will sieze up. They can check by adding florescent dye to the refrigerand and using a black light to find the leak. If it were me I'd get it to a shop PDQ to have it diagnosed and repaired. Avoid using the AC until it's fixed.

Thanks, Tom.  We did the dye test just last week on my son's car, but got off easy with replacing a valve and some refrigerant.

Do you agree with "most every repair shop" that I should replace the compressor as well, even if the problem is the seals?

Yes, even though the compressor can be rebuilt in some cases, it is easier and more cost effective to just replace a bad unit, rather than attempt a repair that may or may not work. Go to a shop you trust or the dealer if they've treated you right before. They'll tell you straight up it's not worth their time or your money. Lots of labor involved in an AC fix. It can get spendy. Good luck

I got the call early this morning:  the compressor is "seizing up" and needs replacing, along with a belt.  If $2,300 is "spendy," I'm there!

Ouch! That'll leave a mark in the old check book.