The amount of air through the vents shouldn't change whether you engage the A/C or not. I'm not sure about your klunk.
Much of the blower can be tested electrically at the three wire connection next to the brake booster. You should get a wiring diagram to see which wire does what.
The blower could easily be the problem, but its hard to get to and rather expensive. There are some easy tests and some easier answers. Be sure that all the fuses are good. There is often a single strip type fuse in a separate small box on the dricers side fender that cracks causing blower failure.
Bosch Master, ASE Master, L1
33 years MB technician