Elusive drivetrain noise

[nvrsunk02]

Guys, I have a 2002 XLT 4x4 with 4.0L auto extended cab. When in 2wd, there is a loud hum, that almost sounds like tire noise, that increases in frequency as i accellerate. At about 40mph, it seems to disapear but that might just be because i cant hear it at that frequency anymore. Ive replaced the fluid in the diff, put on new tires, balanced wheels, checked the bearings (no play) and still no change. Now, when im driving in 2wd, and shift it into 4wd when moving, the noise immediately disapears. However, when I let off the gas and coast/decelerate when in 4wd, the noise comes back. When I accellerate again, the noise goes away. The change is immediate as soon as i hit the 4wd switch. Any ideas?

 

[BlackBII]

Well for starters, have you checked the fluid in your T-case and Tranny?
Driveshafts have any play maybe?
just some suggestions

 

[nvrsunk02]

I have already changed the tranny fluid, but have not changed the Tcase fluid. I also have not changed the fluid in the rear yet, but have in the front pumpkin. I also took the rear driveshaft out and lubed up the splines, which got rid of the notorious clunk. There is a little bit of play in the driveshafts, but how much is too much? I also noticed that the front driveshaft is not original since it has "03 Ranger" written on it in yellow chaulk. I also notice a vibration every once in a while. Its not consistent at any particular speed. When Im on the highway, sometimes its there, sometimes it isnt. Sometimes it shows up, then it goes away. Wierd.

 

[tkdan]

I have a 2000 4x4 and it started doing this a few weeks ago. When I'm at 70+ it is so loud. All I've checked is my front diff and that's all. I'm worried that something is not getting lubed. I have street tires so I know it is not from them. I did not think about the T-case because the sound seems to come from up front, although that can be misleading.

 

[BlackBII]

How many miles on your guy's trucks?
Front bearings need to be repacked with grease
Just another suggestion

 

[tkdan]

Only 130,000 barely broke in.

 

[nvrsunk02]

Ive got about 85k. Front bearing on this thing is supposedly sealed for life. I originally thought it might be bearing too, but when I found out that putting it in 4wd made the noise go away I crossed that off the list.

 

[tkdan]

Front diff, t-case, trans all good on fluid. I dunno!