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2002 GMC Sonoma sounds like an airplane!

This is kind of long, so please bear with me. I have a 2002 GMC Sonoma, 4wd, with about 100,000 miles on it. It recently started making a new noise. When you start it up and drive it, it makes a noise that gets louder as you increase speed. At about 20-25mph, it sounds like an airplane. The noise decreases as you slow down. If you shift into neutral and rev the engine, the noise is not affected, eliminating anything in the engine. When you slow down to stop, you feel a slight sensation in the brake pedal, like the transmission downshifting (this is when you are nearly stopped), and the noise goes away. You can then drive as normal with no noise at all until after the truck is restarted and driven after being shut off and parked. Then you repeat the cycle. Any ideas? The truck is in 2wd when this happens. Thanks for any insight!

Is the truck a standard shift or automatic? It sounds like the torque converter is locking up at the wrong time if it’s an automatic.

I’d need you to break down the whole description better. First, I’m assuming from some clues that this an automatic transmission?

Upon first starting the truck, and just sitting still idling, there is noise or no noise? If no noise when you first start it, when does the noise begin? After it is put in gear and you’re still sitting still? Or only after you get up to X speed?

Then you seem to be saying that you drive it and it makes noise until you stop. Then the noise goes away and doesn’t come back until you shut the truck off an turn it back on. Is that it? How long a drive are we talking about? If you drive 100 yards and stop, then start again is it now quiet? Or do you have to do a few miles first?

What kind of air temps are we talking about? Or does that not seem to matter?

When you clarify all of that provide a complete rundown of the state of maintenance - especially transmission and differential fluid services.

BTW, if the airplane sound is like that of a small, single engine plane (droning), that’s exactly what a bad wheel bearing sounds like. That shouldn’t go away though I once did have a wheel bearing early on its failure that made noise when cold and got quieter after driving a while. The shop speculated cold grease that lubed better once it got warmed up - made sense to me.

Does the truck have automatic locking front hubs? When the grease is cool and stiff the automatic hubs may be engaging and after driving the grease liquifies enough to allow the hubs to disengage when the vehicle stops.

The truck is an automatic. When you first start it up and just sit there idling, there is no noise. It begins when you are moving forward; it doesn’t make it in reverse. You start to notice it as you increase speed - at about 20-25 it is loudest. It does sound like a single engine prop airplane is trying to land near you. Once you come to a stop, the noise goes away and does not reappear until after the truck has been parked, shut off, then restarted and moved. Air temps don’t seem to have any effect on this. I had a complete driveline flush and transmission flush within the past year, so all fluids have been changed. The truck has pushbutton 4wd; there is no manual lock-in of the hubs required. I appreciate any insights that you can give!

Does it change at all when you turn one way or the other?

Does it matter how far you’ve driven before a stop and restart? I.e. if you start the truck cold & drive just long enough to create the noise - like a couple hundred yards - and then go again, does it still go away. I’m just trying to get at whether or not the wheels need to heat up for it to go away.

Turning has no effect. It always goes away after you’re almost stopped, even if it is only a hundred yards or so. I don’t know how much the wheels heat up in that distance, but it is possible that has something to do with it.

Pop the hood and listen. I’m thinkin’ fan blade, but you might be able to find the source just by looking.