Grinding noise from A/C compressor

I have a 2005 ZX4 XT with 11,000 miles makes a slight rumbling or a moderate
grinding noise when the A/C is on, when the engine is idling. The noise
comes on and off periodically, as if from the compressor clutching in and
out.
I don't hear it at speed, probably because it is drowned out by normal
engine noises.
The cooling seems to work fine, temperature wise.

Is this normal? If not, I'll take it in, as it is still in warrantee.

Thanks,
Roger