A/C whine, is it normal??

[Bob Mansker <oscargum@datasync.com>]

Have a 87 5KQ.
The A/C puts out a whine that starts at 3,000 RPM & up.
The whine varies with the changes in RPM.
The A/C is low on freon but is there a reason for the whine?
Is there a sight glass on this Audi?
Where is the LOW side port?
Many Thanks,
Bob