I have a '97 S-10 with big V6 and auto tranny. Somethning has started clunking around up under my dash on the right side. It sounds like it's inside the heater box. It does this periodically and spontaneously. Has a bit of a bellows or diaphragm sound to it, and maybe a flapper valve.
It just does this as I go along, for no apparent reason.
What is it telling me?