This is a weird one:
1) There is a hill about 1/8 mile down the street from my house.
2) When I start the truck in the morning and go DOWN the hill, I hear a loud humming only when the truck is going down the hill.
3) When the road levels off, the noise stops.
4) Any other downhill sections I may encounter on my drive do NOT replicate the noise.
5) Going uphill right after starting the truck does not replicate the noise.
6) Throttle does not seem to affect the noise.

It would seem to me that the noise depends on whether or not the truck is warm and the angle of attack. And I don't know what the hell it could be. Anyone got suggestions?

 

so are you accelerating downhill or coasting/braking? is your gearing determining your downhill speed? got oils to proper levels?

 

1) Mainly coasting, although I have tapped the gas to see if it stops the noise. It does not seem to affect it.
2) Gearing is not limiting the speed.
3) I recently changed the oil (within last couple weeks), lubed the driveshaft, and changed the transmission oil and filter (4 months ago).

I will check the levels when the weather warms up.

 

have you checked the diff level

 

just was thinking cold thick gear oil at startup may not be circulating well if low. but....

bearing noise is usually better heard unloaded like coasting but no change with acceleration, so....prolly not

can you tell if the hum is coming from central drivetrain area or one side or?

 

I changed the diff fluids about 4 months ago as well. Took the front diff apart to re-seal it and everything looked good.

I think the noise is coming from somewhere in the engine bay area, but I haven't been able to locate it yet. I will try to get you guys some more concrete information as soon as I can.

 

you have sai?

 

Negative

 

That's what I was thinking, but I guess not.

 

I had a humming when driving down hill, oddly enough it was my front prop-shaft.
If you haven't replaced / rebuilt the front shaft I'd pull it off and inspect it's range of motion.

DII's a double cardan joint assembly. They're also lacking rather important grease zerks, and located a couple millimeters from the catalytic converter. They get hot, and the centering ball dry's out and... Yeah.

I've experienced u-joint vibration on other vehicles, and my landy was nothing like any of them. It only vibrated / hummed when going downhill. And other than that, had absolutely no symptoms.