I've got a '98 F150 with a 4.6/Auto with about 132k miles. Other then my brakes, which are ready for some attention, the rest of the truck has been maintained very well. For about the last two weeks, I've noticed a fairly strong vibration when on the highway. It is strongest between about 70 to 75mph, and seems to vibrate the worst when going about 70 on the highway, and then completely letting off the gas (such as when coasting or stepping on the brake.) When letting off the accelerator, you can feel and hear a relatively strong vibration. At lower speeds it is almost unnoticable. I can't tell where exactly the vibration is coming from when driving, but I do know it is not coming from the front (engine bay) area of the truck. It seems like its coming from somewhere between the middle of the cab to the back in the undercarriage somewhere. I went to get my tires balanced and rotated just to eliminate that out of the equation which seemed to make no difference. When turning OD off, it seems to make no difference either (if anything, with the RPM's up higher with OD off, the vibration is more noticable since the RPM's drop a lot more when letting off the gas.) It sounds and feels a little like what I would expect a bearing of some type to sound like when going out, although it is not making a howling type of noise or anything like that, it is more of a shuttering/vibrating type of noise similair to what a tire that is not balanced correctly would sound like. At about the same time this issue began, I also began hearing a squeaking type of noise coming from the same general area (torwards the back) when in reverse (only). That noise gets faster as I pickup speed in reverse, and slows back down as I slow down in speed, and then is gone completely as soon as it is put back in drive. I have no idea if the two are related. Any ideas on how to go about troubleshooting this type of issue?

 

STX

I've got a 2000 with sortof the same symtoms The only thing that I could find was a little play in the driveshaft at the front yoke.It is not leaking at this time but I am going to replace the bushing and seal as soon as I can find one. The local Ford dealer will only sell the whole rear housing. Around $150. Mine is a 4x4 so it is on the transfer case.

JON

 

I would suspect you have a bad or dry u joint
the only proper way to check them is to remove the shaft
mitchykins

 

What would the symptoms of a bad U joint be? I am aware of the fact that at 132k, the U joints are at a point where they probably need to be looked at, and had that on my list of things to do, but did not think the type of problems I am currently expierencing would sound like it would be related to the U joints. Am I wrong?

 

a vibration that can be played with the accel pedal
and a squeak -squeak relitive to engine speed usually tells me tht the drive shat is out of balance the squeak tells me
dry ujoint
mitchykins