Got a strange noise coming from the dash of my 04 4dr Blazer.

First off, everything works, ac and heater, all the vents and defrost.

It only does it when the AC is cycling (I noticed it stops when the clutch cycles off), and only at idle (once the RPMs come up above about 1200 it stops)

It is sort of a vibration noise, sort of hard to explain... Coming from right behind the climate controls it sounds like.

I did notice last night driving that with the AC on (only on the lowest setting) coming out of the dash vents, the windshield started to fog up a bit just above the defrost vents, like maybe the blend door is bleeding air past it?

Is this a common issue? Is it just that notorious blend door? I read about the blend door actuator actually going out and stop working completely, but I didn't find anything like my issue.

 

Noises are tough to diagnose, especially online I suppose it could be something with the compressor.

The moisture on the windshield was probably condensation forming on the outside, due to high humidity. Fairly common if the seal on the heat defrost door, or the mode door, starts to deteriorate and loses its seal to the heater case. The notorious blend door issues are with the temperature blend door actuator, and it can not cause what you're experiencing. If the blend door actuator fails, you can't adjust the temperature. It has nothing to do with where the airflow comes out.

 

Yeah that's what I thought about the common problem, if it were my issue I wouldn't have any cool air at all with the AC on, which is definitely not the case, just put a new compressor, condenser, accumulator, lines and all (only thing I didn't replace was the evaporator but I thoroughly flushed it) and even did an Intrepid electric fan install. The AC is ICE COLD, and everything works like it should, the noise is just more of an annoyance than anything... granted with 2000 watts of stereo I can drown the noise out LOL but still I want to figure it out... it'll drive me nuts until I do...

 

Pulled some of the vacuum actuators and all the vacuum lines from under the dash from an 04 at work today (I work in a junk yard; super helpful when I need to find parts ) and I'm going do mess around in there and see if maybe one of those actuators is bleeding air. Also grabbed the 3 outlet one way valve under the hood by the brake booster, just to swap it and see if it makes a difference too.

 

Update on this. I dug in under the dash yesterday chasing this noise down... actually two different noises I've noticed...

Got to the bottom of one. Noticed I could feel it through the gas pedal as a vibration, but couldn't really feel it anywhere else like the firewall. So I chased the throttle cable under the hood and noticed I forgot to bolt the support bracket for it back to the intake when I did the intake gasket job... it was vibrating on the manifold and the vibrations were carrying into the cab where I was hearing them... Bolted it back on, and that noise went away

I'm still left with a lower pitched vibrating/rattling noise though, that my FG has most lady-like called a "farting noise" lol, that I think the AC has nothing to do with because it does it weather it's on or off... When accelerating it speeds up with the engine RPMs for a second or to but then goes away, but only when in gear and acceleration, I cannot seem to get it to do it when in neutral, which is making it a royal pain in the **** to track down lol, but it almost sounds like it's coming from the lower middle of the dash area, behind the ac/heater box. Needless to say, I'm not going to tear the dash apart to chase a noise if everything is working properly, so for now I'm just going to deal with it and turn the radio up an extra notch; problem solved