Worsening occasional vibration when in gear on my F150

Maintenance/Repairs
1

Vehicle: 1998 F150, 158k mi. MT (M5OD), 4.9l I-6.

Description of problem: About 2 weeks/500 miles ago, truck developed a scraping/squeaking sound. I thought maybe something was rubbing on the driveshaft, but inspection revealed no likely candidate, nor disturbed metal on the shaft.

Over the course of the next 500 miles, it’s gotten worse, so that current symptoms exhibit themselves (to one degree or another) 80% of the time. The symptoms are:

  • Worst on first drive of day, on first mile.
  • Sound produced somewhere between scrape and squeak.
  • Vibration noticed throughout cab and through steering wheel ONLY when foot off clutch…smooth as soon as clutch depressed.
  • Giving it a little gas (at constant speed) usually smooths out symptoms somewhat.
  • Recently had bed loaded to gross. Vibrations went away, to return once bed empty.
  • MT fluid found to be full, in good condition, and with 15k mi since change.

So, what’s up? Given that a load in the bed improves things, I was thinking U-joints: but the driveshaft spins in neutral, too, so why would pressing the clutch change anything? I also thought about tired clutch bearings, but everything I’ve read says pressing the clutch makes things worse, not better.

P.S. Is it worth examining the tranny-to-engine bolts?

2

Raise the rear tires of the truck off the ground.

With the transmission in neutral, grab one of the rear tires and rotate it back and forth while watching the U-joints to see if there’s any slop.

Tester

3

I’ve had to replace front U-joints on almost all of my Fords at high miles although when inspected by hand they felt perfectly tight with no slop at all. Once the driveshaft was dropped the rough joint could be felt.

The rears always felt fine but I replaced them anyway.The front ones always had flattened, rusty needles that were bone dry.

4

Yes, this sounds exactly like U-joints. Please note that under power, the U-joints are subjected to different forces than when coasting, so coasting can change things - and is a good clue towards U-joints.

5

Turns out it was the u-joints. Front trunnion, 3 were greasy, 1 rusty.

Thanks everyone for your help!