GM style climate control question

[pj67coll]

I'm investigating Suburbans, the 92 to 99 body style. I test drove on the other day that seemed in good shape but was puzzled by sounds coming from the firewall area under the passenger side of the dash, down by the black contraption that I assume is the blower motor/evaporator box. Although the AC worked fine every now and then varying between several times a minute and every couple of minutes there was some kind of noise like a flap that would open or close with a substantial hissing and banging sound. Sorry, no other way to describe it. Not very loud just enough to be noticable over the sound of the vehicle at speed. I'm guessing that might have something to do with vacum actuation of the air flow in these things. Does anyone have any idea if that might be right or if it could be something else? I assume this is essentially the same system used in all full size GM pickups of that era? If so would replacement of this unit be a problem?

- Peter.