The first time I really "used" my A/C (i.e., extended operation at relatively high speed ~ 60 mph), I noticed a sound every time the clutch would engage. It was kind of like a small low pitched screech.

Once I arrived home and peered under the hood, I noticed that the compressor belt tensioner accessory was missing its nut. This is the little screw that you're supposed to turn in order to adjust the belt tension. The tensioner accessory was partially engaged into the slider rail, ...but the retaining nut is gone. The A/C belt is tight and not loose at all.

I didn't run the A/C and try to rev the engine to duplicate the sound because I was ready for supper. Do you think that the low pitched screech sound every time the clutch engages (at high speed) is due to this tensioner nut missing? I guess what I'm asking is ..assuming that all the other bolts/nuts on the compressor are in place, would the tensioner accessory still be necessary?
Thanks,
Cabbie2169
p.s. This is a '93 245..I have the R134 system