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2002 Nissan XE Frontier--what's this sound mean? [diagnostics by noise]

just purchased nissan truck from a first owner. i hear what i interpret to be some servo-controlled vent, or valve that seems to be opening or closing making a squishing sound, like a rubber seal might make in a tube if it were worn, or needed some lube. the sound seems correlated with the cycling on and off of cooling mode of the a/c. the a/c cools well; but this sound happens intermittantly, once or twice a minute perhaps (i’m in texas during this test, where temps have been over 103 for 61 days running!). nothing seems to malfunction, but the sound is a nuisance, noisome you might say. wot th heck is this? i keep wondering–while thinking that whatever it is, some part must be rubbing and wearing against another part, and this wants preventive maintenance. there should be no ducting or valve-werk sounds audible all the time in the conditioned cabin.

someone familiar with auto a/c setups might know what this soft but repetitive sound is, or might have had the symptoms and discovered the problem and solution.

any help appreciated.

thanks, michael