My 85 westy has a slow leak (as many do) and so about 3 years after
my last R12 chargeup, I decided to go for a R134a conversion.   The
shop (Harris Radiator near San Diego) did a great job, and even
offered me a guarantee, offering to change back to R12 if I wasn't
happy with it.

Performance-wise, it's decent and I think livable.  Not ice-cold, but
quite cool, and I like the idea of not leaking freon, so I think I'll
stick with it.

The only problem that I've noticed is that when running the AC at
high engine rpm (3000 ish) the compressor is making a "hum" sound.
Loud enough to be heard over engine & road noise on the freeway.   I
can't remember if it made that sound while running on R12, since it's
been a while, but I don't think so.

Is it possible that R134 is putting more stress on my compressor and
hence the noise?  If so, is this the sound of progress or incipient
failure?