Hey guys, my truck just started making a rapid clicking sound coming from the A/C compressor whenever I turn it on. As soon as the A/C is turned on the clicking happens, and goes away as soon as the A/C is turned off.
The air still blows cold.
The speed of the clicking goes up and down with RPM - only noticeable at idle, and just off idle (you can hear it ramp up with rpm as you take off, but then it seems to disappear above 1500'ish rpm).

The radiator electric fan is only a few months old, so I doubt that's the problem (Although I haven't run the truck up hot enough -extended idling on a hot day for example - to force the radiator fan to come on by itself)

Question - Is there some type of replaceable 'coil' on the compressor I can replace? Been looking at A/C parts online and can't seem to find a separate coil.
Or does this sound like the clutch itself is letting go?

Compressors are relatively cheap, so I'd replace the whole thing, but if there's an external coil I can replace (without needing to discharge/recharge) that would be awesome.
Truck has 90k miles on her.

Thanks