Defroster weirdness / Air conditioner

I have a 1987 Camaro IROC-Z28 with the 350. I started it up in the garage the other day and let it run for a few minutes. I noticed that it sounded like the engine was speeding up and then slowing down. When I looked at the
tack I could see that the RPM would be at 1000 and then drop down to 700 for about 20 seconds and then go back up. I could hear a clicking sound and figured that it was the air conditioner pump turning on and off. I opened the hood to verify this. The heater controls were set to defrost, the temp was on cold, and the fan was on low. This did not occur with the selector on heat or vent. However when the selector was on defrost, the air conditioner pump would kick on and off at about 20 second intervals. The drivers side fan was also cycling on and off. I was just wondering if this is correct operation. I can't see why the air conditioner should run when the defroster is turned on. I have had the car for a year and have had no problems so far. The car only has 18,000 miles on it. I would appreciate any input.
Thanks, Mike
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Mike Vossler Spilled my beer when they jumped on the table and proclaimed in

The defroster does cycle the AC to remove humidity in the summer.
HTH
NOI
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On any of the cars after, say the 60's, the airconditioner will cycle like that on defrost. This is to help get rid of the moisture on the inside of the glasses faster, than the old fashion type that would put out just heater air.
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