I replaced the compressor and drier last year because the car was making the same exact noises. I was getting paranoid of comp. seizure so I bought new parts.
If I press the defrost or other two CC buttons to the right it will make the noise. No matter where the temp wheel is set. If I press the economy button the noise will go away. Otherwise the system works great.
I was thinking it may be the water pump as more stress is put on it when the clutch engages for the comp. But why would the clutch engage when I have the temp wheel set for heat?

The noise is somewhat like a bearing going bad. Bad enough that you can hear it inside the car. I will try to pinpoint the noise tomorrow.

Any ideas?

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R