Startup engine noise

I have an 02 Blazer w/ 110K miles. It's the 4.3L with 4WD. When I start it in the morning it usually has a low pulsing whirring sound for up to 30 seconds. It then goes away until the car is parked for a long time again, say overnight. It's a low volume sound, a lot like the sound of off road tires running down the highway. This has been going on for several months now.

All systems work fine. The alternator is new, the water pump has about 29K on it, and the top idler pulley has been replaced. Turning the A/C on or off makes no difference. The sound is loudest when you stand in front of the passenger side headlight which made me suspect and swap the alternator.

Anyone ever heard of this? Could it be the belt? It was replaced with the WP and the ribs are not smooth inside. They are notched. I assume that's to improve the flexibility of the belt. If I take it to a shop they'll get 30 seconds each morning to diagnose it. Yikes!! Any help will be greatly appreciated. :confused:

Run the cold engine briefly without the belt... noise, no noise? Check each pulley by hand for bearing roughness, side play, etc. Repair/replace as needed. Replace belt. HTH

Thanks. I'm a bit embarrassed that I didn't think of that myself. I'm now suspecting the tensioner pulley. S/B cheap to try.

Doe's this truck have hydraulic lifters, or cam followers?

No blowers dont know about the lifters. stock 22re.

Suspecting the a/c or alternator after thinking I am leaning more towards the a/c cause I have never heard of an alternator making noise.

A/c could just be coincedense happened now.

This could be the notorious belt noise problem. I have intermittent belt noise upon initial start up on my 2000 Jimmy SLE 4.3L . I have found that a brand new belt will eliminate the problem. I try to stick with the Gatorback belt. You will chase the frikking noise all over the belt drive system, so before removing and replacing ANY belt driven component, take a good look at the belt first. If your belt is more than about 2 years old, replace it for good measure and see if the noise stops. Running the engine without the belt and Spinning bare pullleys by hand is a good idea but won't reveal the problem if it is the belt to start with.

OOPPs! Sorry wrong thread. Not sure how I did that.
Sorry guys

Possibly the AIR, (Air Injection Reaction) system, sometimes called secondary air. Pump is on the back of the alternator, maybe one of the check valves burned up or a hose came off. Just a thought ;) Is the SES light on? Do you have any DTC's in memory for secondary air?