Strange F-150 Vibrations

Here's one I haven't run into before.

I have a 2001 F-150 4X4 regular cab, long bed, V-8 w/5 speed manual tranny. Mileage is 31K

I get an intermittent vibration running on the "sort of" smooth roads we have around here. When it happens, it feels like the whole vehicle is shaking at wheel rotation speed.

The vibration doesn't come up the steering column, can't feel it through the shifter or accel pedal, so I think it's coming from the rear end.

If I push in the clutch while rolling, it has no efect so I don't think it is associated with the motor firing unevenly or loose motor mounts.

The weird thing is, it tends to come and go. If it starts, it will start gradually and build to the point the mirrors are shaking, and I pull of the road. Typically what has happened then is I'll make a turn at slow speed to pull off. When I pull out again it's gone - for a while. This has been happening about 2-3 times a week for about a month. I took it to the Ford dealer and asked them to check for a loose wheel or disk, nothing found.

It doesn't sound like the growling associated with worn driveshaft u-joints, the sound is very dampened. It's more of a 'judder'

Anyone with ideas?

Thanks,

Jim

Calamari - Thanks for your reply - but mine's a manual transmission, and, if I push in the clutch, it has no effect so it's not related to the tranny.

I have heard of problems w/the automatics.

Thanks,

Jim

Thanks guys for your ideas.

It's kind of strange - it only happens infrequently. I can't make it happen intetionally, and can't predict when. This week on Tues and Wed once each day then nothing since.

I'm wondering if there's some water in one of the tires causing an imbalance that comes and goes. Heard of this before?

The drive shaft idea is something to explore further. I've arranged to have the service leader at the dealer drive it for this weekend to see if he could identify it, but, unless it happens for him I doubt I'll get any satisfaction.

I don't want to wait till is screws up all the time - I'm driving 400 mi to Hatfield McCoy in WV next week for a 4 day ride and don't want to get stranded!

Guess I'll just keep the fingers crossed.

Thanks again

Jim