I bought my current truck this past December (2002 SCREW Lariat FX4) and recently had my first reason to engage 4WD. I noticed a rumbling noise at speed (35-40mph.) After searching this forum, I checked the heat shield near the front drive shaft, and it is not rubbing. Everything looks to be in good working order. The sound is coming from somewhere in the front driveline, not the transfer case, as it the sound continues after the t-case is switched to 2wd, before the center disconnect disengages. Checked the front diff fluid and it is topped-off. The drive shaft has some significant surface rust, so it could be the u-joints, but it's a pretty straight shot from the t-case to the front diff, so I don't think the sound is coming from there.
Does anyone have any ideas where the noise is coming from? CV joints in the half shafts, perhaps? Truck has 106K miles on it, and is in immaculate condition. I doubt it's ever seen any serious 4WD duty, but it did spend it's first two years in the Detroit area, so it may have seen some winter highway use. My last truck was an '01 4x4, and it was always whisper quiet in 4WD.

 

vacuum cylinder on the pumpkin disconnects the power at that point. The half shafts turn all the time with the wheels. I think i would jack up the truck and do a little rotation of the drive shaft and "look". YOu might be able to switch hoses on the vacuum cylinder and it woul engage the front axle and drive shaft and turn them (from the tires backward). The electric motor at the transfer would still be off... Might help narrow donw the problem.. Or maybe you can disconnect the motor plug and still engage the front end to test...Does it do the same thing in LO and HI ?? Different gears inside the transfer case. Also, pull the hoses off the axle pumpkin so it cant engage that. Put in 4 HI and let the transfer case motor engage and spin the drive shaft -- without the front axle.

 

Thanks for the response. I like the idea of trouble shooting by trying to get the front end to engage with the shift motor for the T-case unplugged. Same noise is present in 4HI and 4LO.
You're right about the axle shafts spinning regardless. Guess that narrows it down to either the R&P in the front diff or the driveshaft...or a bearing in the front of the t-case...or???
Will try to get some garage time in the next couple of weeks and report back.

 

IF you think its in the front end, disconnect the two vacuum hoses and let the transfer case drive the drive shaft, but not engage the pumkin.