Defrost And Clunking Noise

[sansonekm]

I've got two separate problems I just haven't been able to look around and try to find the answer since it's been raining the past 7 days straight...

First problem is that when I change my controls to defrost everything still blows out of the front vents and not the defrost vents. I plan on rebooting my computer once I get a chance to see if that fixes the problem.

Second problem is after I'm driving and I get to a stop light once I've been in an idle for about 20-30 second I'll hear/feel a clunk noise below me sorta in front of where the seat sits. It makes the noise once and then it won't make it again till a start and stop again.

I'm just looking for some possible ideas on what to look for once this never ending rain lets up.

 

99 GMC Sierra 2500 2WD

[jayclover]

Can't help much on your noise issue (sounds drivetrain related though...imho), but the problem with the vents may be a faulty HVAC Vacuume Actuator. It's a door that controls where the air flow is directed. I had the same problem in my '01 Silverado last year. And it seems to be a pretty common occurance. Definitely a DIY job...

[txab]

I'd check the slip yoke/spline joint. Make sure its lubed so it is able to move freely