1998 5.0 AWD weird noise, help please!

[cconcepts]

Last weekend I bought a 98 5.0 AWD XLT. The guy I got it from removed the front driveshaft saying it was making a weird noise...Ok I thought, so it'll just be 2wd for now. Heres the thing and what Ive done so far. When driving from a stop, theres a thump / grind sound coming from under the vehicle. It sounds like its in precise timing with wheel speed (once per revolution). Becoming a steady moan after about 30 mph or so. While driving whenever I let off the gas, the sound turns into a higher pitched moaning sound, press the gas, back to the original sound and vice versa. I jacked up the rear under the diff uintil the rear wheels were off the ground, had my wife start it and put it in drive, holding the brake to allow the rear wheels to just barely spin. I heard the noise, put my hand on the diff...felt the noise (it seemed) coming right from the diff. Dropped the rear driveshaft, drained the diff fluid. with the rear wheels in the air and the driveshaft off, I spun the rear wheels by hand faster than before...no noise. I thought at first it was my pinion bearing in the rear, but now I have no clue.... All my u joints are good, no play whatsoever. Now I pointing a finger upstream on the driveshaft maybe to the transfer case? Please give me some ideas guys, I dont wanna be driving down the road and this thing blow up on me....thanks in advance!

 

[ericautopart]

Sounds like a pinion bearing to me. Why'd ya rule that out? Very common and I've had to do it twice on my 98 XLT 5.0 AWD. I've got 115,000 miles on it too so it's not like I've got 300,000+ miles like some of these guys. Also I'd replace the front drive shaft..not because I'm looking to make a sale but because your truck can roll while in park w/o it. The e-brake on these things are very strong and it's VERY common for the cable to rust rendering the brake useless. Good luck!

 

[cconcepts]

Thanks so much man, finally someone responded! I actually did another test today on it and I have once again pointed at the pinion (me spinning the rear wheels without the driveshaft connected had NO load on the pinion bearing). So I have the bearing and will tear into her this weekend. Thanks also for the front driveshaft thing, cause I had my truck on my driveway this past weekend on an incline, and about an hour later, the rear wheels were in the street!...lol So thanks again man. I will update after bearing is replaced.