99 tahoe cabin noise - sounds like vacuum

[osukellen]

As a matter of fact yes, about 3 months ago, right before I did the spider upgrade. During that time though, it was idling a little rough so I may not have been able to hear it until I got the smooth idle back. Wonder if we got bad wires? Mine weren't cheap, but they were top of the line either...

 

[718frankb]

i did the msd wires. i think we have electrical interference ! time to start seperating plug wires,cleaning grounds etc

 

[ravingmadman]

Your recirculator won't come on in certain modes of air direction. I can't remember all of them off the top of my head, but I believe it won't come on if you have the defroster on. Screw with it. As far as a sucking sound, pop the hood, and open your door. You need a quiet place for this, so if you can do it in the garage (with the door open, Dr. Kevorkian) that's the best. Start your truck, let it idle for a couple seconds, then turn it off. Roughly a quarter second after you turn it off, you should hear your intake take a breath as the vacuum dissipates. If it's the same sound you're hearing from behind the dash, then you know you have a vacuum problem. That's the poor man's way of doing it.

You can also put a vacuum guage on the intake manifold, and if the needle bounces, you know you've got a problem.

The HVAC control panel is operated by a bunch of wires as aforementioned. There aren't any vacuum lines behind the radio area that I'm aware of. Hope that helps!!

 

[spicewood1]

Thanks to all so far. Yes the vacuum sound does continue after engine shutdown, dissapating to nothing in about 3 seconds or so. I really can't believe how audible it is inside the cabin - it honestly sounds like an old Qjet. The recirculate does not work in any mode of operation, I tried everything. Its not just the bulb either, my other '99 you can actually hear the system change sound when the recirc flap shuts and this one makes no change. As far as i can tell there are only 3 vacuum connections off the intake - the brake booster, a tiny hose going to the firewall near the pass side, and a small hose going to what looks like evap stuff near the driver rad support. Disconnecting and plugging any of the 3 make no change to the sound. Maybe I am missing one?

 

[spicewood1]

Update - hold the phone - I realized the recirculate is indeed working, its just not as audible on this 99 as my other 99. The indicator light is intermittent so that is not the issue.

Oddly enough I did notice that holding both front window switches in the up position as the glass is already up, thus loading the charging system, makes the noise go away totally and instantly. It does make the engine rpm dip maybe 50 rpms so it could be either the lower engine speed or maybe the alternator. Will swap out alternators tomorrow and update further.

 

[spicewood1]

Not the alternator. I swapped with a known good unit off my other '99 and no change.

 

[osukellen]

Check the distributor.

 

[Geardaddies]

Spice, have you figured out this problem? I have had this problem since rebuilt engine and injector upgrade.

 

[oldsalt]

Follow your fuel lines down the back of the engine and make sure they are not touching the body seam at the firewall. The clearance there is really tight.
If they are touching, loosen them a little. I put a small pry bar between the lines and the body to hold them away from the body then re tighten.
I also put a small piece of air hose around the line just to be on the safe side.

 

[Geardaddies]

oldsalt, did you have the same noise and doing that to the fuel lines helped eliminate the noise?