Two months ago I posted a thread for help with drive line noise on my '04 Exp. 4x4 now w/98,000 miles. This model also has the auto 4x4 selector as well as 4x4 high and low. I first heard it 10,000 miles before that so it was getting worse. You guys were very helpful suggesting front Hub assembly as the culprit as noise seemed to be on right side. It has the classic rear axel bearing sound from the drive line that my old Fords had. Starts as road noise @ 40mph similar to noisy tires and it is constant no matter if coasting in neutral or driving under load or not. The pitch varies with increased RPM only. One difference is that when you go over a dip the wine noise changes a little.
It appeared to me that it was going to be harder for me to fix so I had both front hubs done by the shop that I had success with before. I bought the hubs, National, made in USA. Also bought seals for front differential axel shaft housing. He charged me labor of $100 per hub and $75 additional to replace one seal on right that has the longer axel shaft tube. So I now have $750 in it. And after it was repaired it still has that driveline noise but not quite as bad, but sounds the same. He only did the one seal on pass. side as that one was leaking just a little.
I took it back to the guy so he put it up in the air and listened with steathascope to rear thinking fronts were changes so must be rear hub. But it was quiet in the rear, just like when I checked it 5,000 miles ago. We put it in 4X4 and he could hear the noise coming from the front differential case especially where the passenger axel housing is- from there to the differential inspection plate. Should I suspect that he forced out the axel shaft from the housing to replace the seal and the spring clip fell out into the diff housing??
His expertise is all type of gears and repairs of rear ends and 4x4 stuff including posi etc.
He believes that the problem is in that differential and let me listen to it. Even at idle you can hear slight turning noise that you can't hear at the rear or drivers front side. But this is a big job as according to him the whole front differential has to come out. He is going to give me an estimate tomorrow but I shutter to think of what it will cost. Noise isn't that bad if I wasn't a mechanic type of guy. So to me it is annoying and bugs me to think it will probably get worse.
I also heard a noise in the rear differential right where the drive shaft connects. But he didn't seem too concerned about that.
Any ideas??

 

Since the pitch changes with RPM and is also noticeable when idling, then it is not a differential noise it is an engine related noise. Pop the hood and listen to the idler and tensioner pulleys for the noise.