When A/C on, strange "burrruuooooop" noise from under dash.
 

1991 N/A coupe:
When A/C compressor kicks in, there is a strange "burrruuooooop" noise that comes from under the glove box. This is, of course coming from the exchanger under the dash.

What the HECK is this noise?!

Sometimes, it catches me off guard and scares the BEJESUS OUTTA ME!

(Its funny watching 1st time passengers when they hear it coming from thier
knees!)

My GUESS, is that it is the expansion valve for the air conditioner and you are underserviced with freon or whatever you want to call it. Key word is my second one. There are at least two air conditioner service pros on this site that I'm sure will give your post some thought. Have fun.