Hi, 2000 silverado ext cab long box , 4x4, 5.3, 200,000 kms.

From August till it got too cold here in Canada for A/C, i had a blow-off type noise occuring whenever I had the A/C on. It would be a short (1-2 second) blast of air, that sounded like a blow-off valve or sorts, that happened every 20 seconds or so. It was more frequent when revving higher, less frequent at idle or low RPM's. I did some research at the time, and I believe I had figured it to be some sort of pressure equalization valve. Didnt bother to fix it, just kinda got put off. I hardly use A/C anyway.

However, today, started getting it again, but not on A/C. Its happening anytime the blower motor is turned on anything but off. Doesnt matter what the heat setting, which vents are on, what speed the fan is on, whether or not A/C is on. Dont think I tried Recirc though, should have. Its about every 20 seconds, a short blow off sound, sounds like its coming from teh center of the dash somewhere (inside of course, towards the firewall. Possibly more to the passengerside.) Heat still works fine, everything seems to work as normal, except for this annoying noise. I cant seem to find the info on that valve anymore, although I thought it was only involved in teh A/C system anyway. I've searched, and cant seem to find anything pertaining to my problem. It goes away as soon as you shut off the blower motor.

Any ideas??

Thanks guys,

Evan

Evan[/quote]

I would drop the cabin filter and the blower motor and inspect for deBris...