I have a 1986 F250, 2wd, with a 351 Windsor and a C4 transsmission. Once the vehicle is warmed up, and I'm driving down the road, if I suddenly slow the truck, I get an awful knocking vibrating sound coming from the engine copartment that stops instantly if I hit the gas again. I thought it was the brakes at first, but it isn't; the brakes are fine and the noise occurs whether I hit the brakes or not. The only time it does it is when the transmission is downshifting and it stops as soon as I hit the gas again. What could it be?

 

fyi that truck would have a C6, the C4 wasnt offered in a fullsize truck.

I would suspect bad ujoints or possibly mounts. check the motor and tranny mounts for wear.

 

Thanks for the speedy reply... but I'm an idiot.

While driving around with the window down today, I finally realized that the noise, which, from inside the cab, sounds like its coming from just in front of and below me, was NOT coming from the front of the vehicle at all.

The damper on the rear axle is way loose, and when I wiggle it, makes the exact noise I keep hearing.

And I have no idea where I got C4 from, I've honestly never ran the tag's ID.

 

what damper on the rear axle? do you mean the sway bar? or rear driveshaft ujoint?

 

Doesn't really look like a sway bar; my Chilton's and Hayne's call it the "rear axle damper," and thats about all the info either one has on it. It sticks up off the passenger side of the diff, has all kind of linkage, and apparently reduces vibrations coming from the axle. Whatever it is, it has about 3/4" of play at two of the joints.

 

can you post a picture of it?

 

I'll post a pic as soon as I get back to the garage; I'm stuck on a 24 hour shift at the moment. At this point, I don't even know if its a damper; whatever it is it's tied into the brake system and connects to the differential through two arms.

I think I've discovered the true problem, though... When I jacked the rear end up to investigate the "damper," I noticed the differential on the 10.25 leaking. Pulling the cover, I was greeted with the horrible sight of a chewed-up differential pinion shaft. Turns out it had sheared off the lock bolt, and was sliding out about a 1/2 inch when not under load, but still moving. That 1/2 inch was enough to bang against the pinion gear.

So, it looks like I'm off to the junkyard tomorrow to find a differential in good shape.