AC compressor whining

[gl1674]

Is it normal for AC compressor to make whining sounds?
Mine makes whining noises when rpms are above 1200 and
the temperature outside is above 80 degrees? The sound is
like if compressor is pumping a mixture of fluid/gas...
No unusal noises on a cool day or at idle speed.
It's an R12 system, I don't know how much charge is left in it,
the window on filter/drier is always clear - no bubbles at all.
It seem to cool OK, but only works really well at highway
speeds (with RPMs continuously above 2500).

Any ideas for the source of the noise?