OK, here's a fun intermittent. The sound is what I would describe as a rotational grinding sound (gee, a chisel on your shop's grinding wheel might be a good analogy). Starts at 60-70mph, but it takes anywhere from 30 - 50 miles of driving before it starts. It will quiet down as I get under 40mph, does not come back if I try to run high rpms at slower speeds. Thought of a wheel bearing as the right front had a small dent in it, race was fine. replaced the bearing and still hear it. 4Crwaler isn't sure either, we've bantered about a couple of ideas. It seems like it comes from the front, but sometimes I can't be sure, maybe even rearward. Some upcoming work is to include a water pump (it's definitely starting to screech at first startup, I've seen some coolant at the weep hole on occasion), but with 170K on the clock it makes sense to go for the timing chain as well. There's also some u-joint issues where the rear one won't take grease, and the fitting won't come off, so likely some new u-joints. So....any other thoughts on such a noise? It certainly seems more drive train related than just engine rotation, but damned if I have isolated it yet. One school of thought has been that if it's so intermittent things ain't falling apart yet, so just play wait and see....

Could it be a axle linkage bearing, like the ones on the extended cab rear drive shaft? THere is one on the front IFS 4x4's as well. any chance a clutch could cause that?
How about a fan clutch or AC clutch? Even an alternator bearing? Something related to higher speeds though....any way, those are my shots in the dark.

That almost sounds like you need to have the jackstands, jack, and a buddy handy. Drive the truck until it starts acting up, then get it in the air and drive it while one of you looks around....

What kind of hubs do you have up front; still the Aisin manuals or ???

Clutch was recently replaced, using parts from Marlin. I had been wondering about the fan clutch, alternator, power steering, etc. I guess when everything's apart for the timing chain it can be poked and prodded, looking for something out of whack. A while back the idler pulley bearing in the A/C went bad, but that caused a noise at all times. Thanks to 4Crawler to helping me get that replaced; I also changed the power steering idler pulley at the same time.

And yes, still the OEM Aisin hubs.

I enjoy a good mystery, just not in my rig!

I had the same exact problem a year ago. Nobody had any ideas at all, I knew it was coming from the front end so I started replacing components. It eventually stopped after I replaced the idler arm. I had noticied that it would start to make the noise after going over a bump and that is I hit another bump it would go away. It doesnt make a whole lot of sense as to why it woudl be caused by the idler arm but that is what made the noise stop for me.

I had a buddy that had an explorer do the same thing. 60-75 crazy grinding whining noise. as soon as the car dropped below 40mph exactly it went away every time. It started after doing it after his wife was doing like 45-55mph in 4low. Then it was really hard to get it into 4low without it grinding real bad. I thought it was the transfer case, but he said the hell with it and traded it in. so I never found out if that was it....

but does your 4x4 still work smoothly??? low and high.
might be worth a try <img src="/forums/images/graemlins/scared.gif" alt="" />

What happens if you push in the clutch and coast when the noise is occuring? Any change? Any difference in the noise between 4th & 5th gear?

If the noise is still there when coasting with the clutch in, I would suspect a drive train problem. I would drain the fluid from the tranny & t-cases and see if there is any metal in it on on the drain plugs.

I agree. does the 4wheel drive still work?

I had some x-fer case issues last February. Rocky at Marlin's looked it over, drained the oil, did not find any metal particles, gave me a clean bill of health when he was done. I had changed the tranny gear oil around that time, too.

I haven't had it in 4wd lately, I'll throw it in and see if there's a noticeable effect.

I have tried putting in the clutch on the freeway when I hear the noise to see if there's a difference. Nope, not really affected by that.

I was wondering if it could be u-joint noise? As I said the rear one is going to have to get changed out as the fitting won't take grease, and I can't get it off. The front one takes grease just fine on one fitting, the grease oozing out of the u-joint where expected, but the other front fitting only seems to take grease in the main channel. It will ooze back out at the fitting when the grease gun is attached. I can remove that fitting, and when I do the grease oozes out of the hole. I'm thinking the channels down into the joint itself are blocked. I read where the #1 failure for a u-joint is lack of lube, but what are the symptoms? I don't get any vibration/shake, just that 60mph+ ocassional grinding. Can that be the joint? I'm not sure exactly how they work, there's needle bearings in there, but can they do that? A rear end shop told me they have heard of noises being caused by u-joints failing.....

OK, I just got home and put the rig in 4wd and ran through all the cases. Everything seems OK. It's a dirt driveway/trail around my place, so there's even some rocks to put a tire upon. I put the left front on a boulder; couldn't have done that if the front wasn't in 4wd, I would think. Everything went fine (thankfully).

Just wondering about my u-joint question in the last post...so if anyone wants to pipe in....

TIA