strange heater noise

I don't claim to be a car god by any means. I purchased a 1995 Astro used a couple of months ago and now that it is getting cold I have noticed a noise. It sounds like a pump or motor cycling off and on. This only occurs when the heater/ defrost is on. Is this a normal thing or something to be concerned about? The van has a 4.3 liter with 104,000 miles on it. Thanks for any insight.

In the defrost mode the A/C compressor is engaged
& what your probably hearing is it cycling, normal.
In "HEAT" or "VENT" this should not happen.