Hello, all. Here's a tricky one .. for me atleast.
I have an '85 F150 4wd short bed. 300/I6 4 speed OD TOD transmission. Lately it has started an occasional clunk that I know is a radius arm bushing. No big deal there.
However the other day I hit a bump in the road going about 45, and the truck suddenly started vibrating and it sounded as if I were driving over a rumble strip. Came to a stop and looked it over, found nothing obvious, and continued on. Nothing happened for about 2 weeks, (Ran on plenty of rough roads in that time) until yesterday. Coming over a bridge, same speed, hit a soft bump and it started up again. The sound remains until stopped. Once you get going again, there's nothing..
Wheel bearings, U joints, and all 4 corners on jacks and yank on anything that moves, all tests out tight and smooth.
This is accompanied with a noticeable vibration around 45-50mph in OD, and in 3rd or 4th at exactly 71mph it vibrates again, and goes silent and smooth again at around 74..
The vibration is obviously something out of balance or loose.. cant figure what though. But this frontend grinding has me a bit weary as it's the only vehicle I drive. It's almost as if one of the front hubs were locking, and running through the diff. But that shouldn't vibrate.. nor happen at all.
Any ideas?

 

Bad shock, just 1 and that wheel is bouncing as the harmonics are just right.
You speed up and that changes the frequency of the harmonics and it stops.
Dave ----

 

Sounds a lot like the death wobble that Jeeps and other solid front axle vehicles get. Our trucks aren't quite solid up front, but I'm not surprised they can have some similar issues.

Death wobble usually is a result of worn suspension parts -- it can be almost anything as the axle just needs a little bit of play for harmonics to take over.
It would be a good idea to fix the radius arm bushings and go from there. Jack up the front end and see if you have any play in your tie rods, ball joints and wheel bearings.

 

Just remember anything with the suspension/wheels will be a slower vibration. The driveshaft turns around 3 times faster than the wheels, so try to keep that in mind when troubleshooting. Try to figure out if it's fast like the driveshaft or slower like a wheel/suspension problem.

 

Partially locked front hub can give you the "rumble strip sound"... and a "clunk" too....

 

I actually had the hubs locked earlier today and forgot to take them back out .. the ride home was awful quiet till I realized what I was doing different. The highway vibration has started quite a while before this sound. I'll have to look into that. But I think you've got the hub noise down. Ill unlock it and try another run in the morning.

 

A busted/week spring can cause it, along with any old grease getting cold, I was told yars ago to use only white lithium grease in the hub mechanism, NOT wheel bering grease...