Working with a 2001 4x4 Ext Cab 6 speed with 185k. No mods. Truck makes an awful sound from 1500 rpm on up when going through the gears. Sounds like a train horn. It doesn't do it at all in neutral at any rpm just when you're accelerating. That should rule out any accessories that are driven by the belt and the front hubs because you can speed up coasting down hill and it doesn't do it. I've had two of these trucks and a 3rd gen and I've never seen anything like it before. Any ideas??

 

Money marble or chalk its your exhaust manifold. Check all of the bolts top and bottom-most likely the rear most 2 cylinders will be hand tight at best. Good luck.
Kyle

 

If that's the case it should show signs of leaking, like black soot around the manifold, correct?

 

Stupid as this sounds..... If you only hear it after you're moving forward, I'd start at the wheels and work inward. You also mentioned, if I understood it correctly, that when going down hill the noise would quit? That sounds like a bearing to me, maybe a u-joint. Check for play in the drive line. When you do, chalk the front wheels and put the truck in neutral, no parking brake. Also jack the truck and check for play at the wheels. You might want to use a bar, like you would when checking ball joints. It won't take very much play to make it howl. While it's up, check the differential fluid levels too.

 

This all started last night and I guess I should have waited til daylight before I got excited. Found a missing turbo hose clamp this morning. Replaced that and it's fixed. The reason it only did it when the truck as accelerating it didn't leak until the boost came up. Thanks for the replies guys.

 

Good news!