Growl-whine From Behind Instrument Cluster

THIS IS MY FIRST POST. SPENT SOME TIME READING THROUGH THE VARIOUS THREADS. IT'S QUITE IMPRESSIVE AND ACTIVE.
TRUCK IS 96 BLAZER 4x4 4.3 AUTO 140K MILES. Noise begins right after startup if it's been parked for at least 5 hours, usually overnight. The noise starts even before moving vehicle. Goes away after truck warms up. Could be temperature related. All guages work ok. Older vehicles had mechanical spedo cables that would whine when cold or out of grease, but I'm pretty sure most newer vehicles have an electronic input to the spedometer. Any thoughts?

Some more explaination may help.
I think the blend door becomes active as soon as the key is on.
Maybe try to turn the ignition on for a minute before you start it and see if the noise is something electrical.

I see your point. 'Does the noise start with just electric - before starting the engine?' The truck is warm right now. I will give it a try tmrw morning. I didn't think a ventilation damper would make this kind of noise but we'll see.

Turned the key without starting it and all was quiet. Noise started once engine was running. Outside temp today was about 50 degrees F. However, after I drove three blocks I parked, shut the engine, restarted it and the noise subsided to about 30% of original volume. Just to provide more detail on the sound- it goes from high pitch to low pitch in a random pattern almost like random morse-code

Do you have the defrost or A/C on when the noise is happening?
When the noise stops is it sudden or gradual?
Im thinking maybe one of four things. 1. Water pump. 2. Alternator. 3. A/C compressor. 4. Power steering pump.

First thing to do is pop the hood before starting. Start up and immediatly run to the engine compartment and start to listen. It is absolutely imperative to pin point the local of the noise.
Without actually hearing it myself its difficult to imagine. Most likely suspects are bearing failures, somewhere.

I guess I would start with the basics. Make sure all fluid levels are proper. Find the direction of origin. Then start individual inspection of suspect components.

Check your vacuum lines in the engine bay, especially those that go to the vacuum ball for the Heating controls.

Check your vacuum lines in the engine bay, especially those that go to the vacuum ball for the Heating controls.
:iagree: Sounds like a winner. The line may be off under the dash and that is the whine you hear is the air sucking through a vacuum line or as BlazerLT said it's off at the ball or your line has a hole in it.:2cents:

The noise is definetly in the dashboard not the engine compartment. It goes away gradually as the engine and interior warm up. The noise happens even with the AC/HEAT/fan switches all off. By 'vacuum ball' I presume you mean the one mounted to the underside of the hood near the hinge on the driver's side. It's connected because it fell off three weeks ago when the windshield was changed. It was an easy fix. No, the noise did not start after the windshield was changed. The noise started several months ago.

The idea of a small vacuum hose leak sounds plausible. If its behind the dash, I not sure I'm want to take the dash apart. I'm going to look at the visible vacuum hoses at the firewall and see what can be seen.

Thanks for the help so far. I wish I knew about this site years ago. I've had to figure out all my quirky BLAZER problems on my own all this time.

I had a very similar issue right after I had my lower intake done. I thought it was coming from the dash, when in fact it was the engine compartment. Two areas to check. Make sure the PCV valve/hose is snug in the port. Also follow the other end of the line to the connection going into the plastic intake cap. There is a 90 degree rubber elbow there. Mine had stretched partially when the dealer reinstalled it. I simply pulled the rubber elbow off, reversed it and wrapped a zip tie around it to make sure it was snug. Problem was gone. It sounded just like a vacuum leak.

If youre absolutely certain its not in the engine compartment than I would think you will have to get into the dash to take the troubleshooting further.

I had a very similar issue right after I had my lower intake done. I thought it was coming from the dash, when in fact it was the engine compartment. Two areas to check. Make sure the PCV valve/hose is snug in the port. Also follow the other end of the line to the connection going into the plastic intake cap. There is a 90 degree rubber elbow there. Mine had stretched partially when the dealer reinstalled it. I simply pulled the rubber elbow off, reversed it and wrapped a zip tie around it to make sure it was snug. Problem was gone. It sounded just like a vacuum leak.
I had the same prob with my Malibu but it just came part of the way out and made a funny squill sound like to never found out what and where it was.

Yesterday I took another look. SWALT hit the nail on the head! It was a loose PCV valve. Apparently the noise travels through the vacuum hose to the brake booster and the booster acts like a megaphone sending the sound to the dashboard. The whining cat behind the dash is gone thanks to AF's helpers. Thanks again.

Boy am I glad I didn't have to take the dash apart.

No problem. I'm a stickler for squeaks, noises, etc. Anything like that drives me nuts as I drive. Glad I could help.