My truck (a 2000 GMC Sierra SLT Extended Cab 3-door, 4x4 with the 5.3l engine) makes a weird noise. It started happening tonight after leaving my mechanic after having my winter tires put on. Nothing was touched under the hood and other then jacking up the four corners and swapping tires nothing else was touched. I am hoping to find out what it may be before tomorrow evening when he is planning on dropping by my place.

The sound is a quick air burst sound. Almost comparable to the sound one would hear from the air blast when disconnecting an air powered impact gun from a pressurized air hose. It happens when the engine is running at 2000-3000 rpm (that's where I tested it) but not at idle which is indicated at around 600 rpm. Here's what Is stumping me, it only seems to happen when the interior ventilation controls are set to the defog setting and the interior fan is at least on setting 1. Changing the fans speed doesn't seem to make a difference. I have only tested with AC off so I think AC can be ruled out.

I am searching google but so far haven't had any luck and thought I might give this all knowing board a shot.

Any ideas?

Rene

Rene-
It makes the sound one time when you change the vent setting?

Possibly, the vents are controlled by manifold vacuum. Somewhere a hose or connection may be broken... or the actuator leaks during a portion of it's stroke.
Just ideas- nothing based in reality, sorry.
Stew

repeatedly.... random amounts of time between, though that might be attributable to rpms.

Ummm, yeah... what Robotic said.

tanstaafl.

Here's some more thought- but no idea where it's leading...
The vents are probably not activated until a fan setting is selected. Maybe that's why you have to have the fan on.
In the del Sol, for example, the AC isn't actually on until you turn on the fan. Maybe GM links all the climate controls with the fan.
Also, it's interesting that the sound isn't made at idle- when manifold vacuum is highest. Could be a clue.
Perhaps the same style of climate control parts are used in other GM vehicles- a search of GM enthusiast sites on the internet could be helpful to you... say camaros or corvettes? Heck, even the trucks have club sites these days...

The attachment is, the diagnostic tree for fan noise, taken from the gm service manual. Don't know if it will help or not.

Edit: Your description doesn't make me think you are talking about a fan related burst of air. At least you didn't mention an accompanying rush of air from the vents. If so this won't help much.

P.S.
It doesn't look like the hvac system has any controls that run on vacuum. The various doors that divert air through the heating and cooling cores seem to all be motor driven.

No, that's not how air conditioning works -- none of that happens. "Freon" is never vented. Air conditioning is an entirely closed system. The pressure on the high-pressure side is regulated by cycling the compressor. The "expansion valve" is not usually a valve at all, except by a rather technical definition. The expansion valve contains a small hole which results in a pressure drop and state change of the fluid. The hole is sized to match the thermodynamic properties of the refrigerant and "chokes" the flow. The AC works just like the cooling effect of discharging a CO2 fire extinguisher, except the escaping gas is captured and then recycled. The expantion causes the state change, which makes the gas cold. The expansion occurs at the orifice in the "expansion valve".

The noise is almost certainly a leak in the vacuum-controlled heater and vent system. The vacuum is higher at higher RPMS, not lower as someone mistakenly said earlier (it is lower *pressure* at high RPM, which means bigger vacuum). You need to take apart the dash to diagnose this exactly, and that can be a bit of a pain. It might not be so bad in a full-sized truck, but it is a royal PITA in most cars. It can be a failing controller or servo or a cracked vacuum line. Its probably not an unplugged vacuum line because you are hearing the PSST sound, which indicates the leak is intermittent.

Depending on the vehicle, the vacuum is used to either close or open the vents. My guess is that one of these vacuum actuators has a cracked diaphragm.

Its not related to the tires. That's just a coincidence, most likely.

FWIW,

Jim

Freon is never supposed to be vented, but I have seen many cars with high pressure relief valves that can vent the freon (preferrable to detonating a random ac component).

The expansion valve is a valve. Usually it's actuated by a bulb attached to the piping on discharge side of the valve and keeps the freon from freezing, it regulates the orifice and keeps the downstream temperature constant. I've had this freeze shut if there's any water in the freon loop. Probably the valve can't make the noise heard.

The high side pressure is regulated by a combination of the expansion valve and cycling the compressor.

The description of the sound being like escaping air leads me away from thinking it has anything to do with the vacuum system. Vacuum never makes sounds like that.

What can make a sound similar to that is a failing compressor clutch. When it engages at high rpm it may be slipping excessively and making the sound. That should be easy to track using a short hose and listening near the clutch.

P.P.S
This attachment shows the schematic including the motor/servos that drive the various controls in the ventilation. No Vacuum used at all.

Kinda pointless now. The sound suddenly went PFFfffffffttttt.... last Wednesday and hasn't returned since. Really makes troubleshooting difficult.

The truck is behaving normally though. We had to go on a 3.5 hour + round trip last Thursday and we were towing our 23 foot travel trailer for the return portion. The truck behaved as it normally would and there hasn't been the slightest indication of any noise since.

I'm wondering if since the truck was sitting for a few weeks prior to this noise maybe a valve got stuck and finally freed up after being used.

The A/C definately appears to be off when in defrost mode (needed this mode this morning btw, -7.5 C) as the light toggles on and off and there seems to be the sound of A/C engaging. With my CR-V when you switch to defrost the light comes on and you have to manually turn it off.

Thanks for all the input though. At least now I know where to return to get some good resources on the subject!

Rene

P.S. You guys were much more helpful the the GMC Sierra Truck Club on Yahoo!