2004 F150 5.4L FX4 with Automatic Trans and 169000 miles:
It is making a loud clunk or more like "BANG" while driving in traffic. So loud in fact I`m thinking it has to be causing some damage someplace in my drive train. As you know in traffic you constantly need to adjust your speed as traffic slows down then accelerate a little to keep up with it as it speeds up again. I am talking about city driving and not highway. Upon pressing the accelerator to increase speed a little, sometimes the transmission will down shift (a normal operation depending on the amount of speed you need) and then "BANG"! The sound seems to come from the rear of the truck. I had a transmission shop test drive it just to see if it might be a transmission problem and the technician said the trans is operating normally. He mentioned that in the past he has had to grease the spline on the drive shaft with Teflon grease to get a bang noise to stop but he never heard one this loud or behave this way. It happens at any speed, all that is required to duplicate the noise is to slow down about 5 of 10 mph then press the gas enough to cause a down shift of the trans. It happens on turns too but I don`t think the turn has anything to do with the noise as much as pressing the gas as I begin to come out of the turn. I got under the truck yesterday to check the U joints and they appear very snug with no play, so the noise couldn`t be coming from that. While I was at it, with the rear wheels off the ground, I rotated the drive shaft back and forth and there is a metallic clank sound coming from the differential. The rotation amount would be about a few degrees and it seems there might be to much lash between the gears inside.
I would appreciate any input anyone might have.

 

Is the truck lowered,lifted or stock suspension?

 

Sounds like a bad u-joint at the rear diff.
The tranny guy should've taken a look at that. I had a 67 mustang that would give me a clunk when shifting into drive and going between reverse and drive.

Decelerating then giving it some gas will release the load on the driveshaft and u-joints, then as you give it gas the load is put back on the shaft and u-joint. A worn u-joint will have slope in it and will give a clunk similar to what your hearing.

I suspect the u-joint.

 

I'd go with the u-joint, too, but you say you don't feel any play. I had a muscle car once that bent a rear u-bolt which made a banging noise as the axle shifted back and forth from deceleration to acceleration. If you're hearing any noise from the diff when you manually turn it, though, that's not a good sign. Any evidence of oil leakage from the diff? Can you drain the oil and inspect it? Looking for metal, of course.

 

Thank you for the responses everyone. In answer to some of the questions posed:

My truck has stock FX4 suspension, just the way it was when I took delivery of it in October 2003.

The transmission guy did get under the truck in the parking lot, he didn`t have a bay available to hoist it up but he didn`t mention anything about the U joints, good or bad. Hej ust looked as puzzled as I am. He wanted me to leave it for a day so he can really look into it.

There are no leaks anywhere that I have been able to see. This drive shaft has no U bolts that could bend like the older fords had, instead there is a circular plate connected to the pinion shaft on the differential that the drive shaft bolts upto with 4 bolts.

The U joints are inexcellent condition with no abnormal play (they are like new) as verified today when I got back under my truck and disconnected the drive shaft from the differential and then I lowered the truck so the tires contacted the pavement and measured exactly how much free rotation of the pinion shaft there was. From stop to stop there is 6mm of free travel. That equates to 5.36 degrees of free rotation.

I`m thinking that as the truck slows, the momentum of the truck causes the ring gear to continue to rotate the drive shaft with the teeth of the ring gear doing the pushing, and when I accelerate slowly without the transmission down shifting, it just catches up without slapping hard against the teeth of the pinion gear. But when the transmission down shifts, there is a slight pause between Drive and 2nd gear and that slight pause allows the ring gear to be seated against the teeth of the pinion gear on the back side of the pinion gear teeth then once the 2nd gear in the transmission takes hold, it`s BANG when the teeth of the pinion gear take over and do the pushing, slapping hard on the other side of the teeth of the gears. Otherwise there is no noticeable vibration or noises during the operation of this truck.

I also checked to see if there was any end play in the pinion shaft. I mean I couldn`t push or pull the shaft in or out of the pig. If it moved I could not tell.

My next task would be to remove the inspection cover on the pig and have a look as Mechanik has suggested.

 

It sounds like you know a lot more about how your truck is built than I do, and your thoughts around the problem are most likely right on. Being cheap, though, I hate the thought of worn or damaged gears. Given that I don't really know the suspension and drive train to the level you do, is it possible that some other component is loose or defective? Sort of a final inspection before cracking the diff?

I'm thinking of the force you get on the mount points of the axle and suspension when you go from deceleration to acceleration. You probably know more about this than I do, but when I used to drag race, I saw tremendous stress on the components that try to keep the axle from rotating when you drop the clutch at high RPM. I'd get stripped nuts, bent brackets, mashed bushings, (broken u-joints too) and anything else you could think of. I've had a couple of 4x4 and never had a problem but I would suspect that a large 4x4 might see similar stresses. but built up slowly over the long life you've had from this truck so far.

Just hopeful that there is something less expensive in your future than a rebuilt differential. Looking forward to your post with the findings from the diff inspection. good luck!

 

Spec on a rear end for back lash is generally between 5 and 12 mm of back lash or free rotation I'd grease the drive shaft splines it works. The rear end sounds to be in spec and if u had excessive back lash you would have a howl on deceleration.

 

I'm still thinking U-joint. When looking at the u-joint could you rotate the drive shaft and determine if there was no slop in the u-joints.

I think greasing the splines is a good suggestion as well.
Is there a grease fitting to do this. A friend has a Toyota 4Runner and they have fittings on the front and rear drive shaft. Actually the drive shaft is a two piece and split in half. At the middle where the 2 halves come together is where the grease fittings are.

 

No you have to remove the shaft and manually coat it i mean coat the crap out of it

 

Thanks again to everyone for the useful info in your replies.
I think I am going to pick up some Teflon grease on the way home from work today and grease the spline to see if that helps. It is allot cheaper than a differential rebuild. I know because this one has been rebuilt back at 78,000 miles. There is no howling or any unusual noises when I decelerate coming from the rear end so I am curious to see what the grease will do.
Also on this website there is another post about the same issue that I didn`t know existed when I posted my issue. Do a search for "Clunk on the downshift". In the threads someone suggested having the computer reprogrammed, so the person who was having the problem did that and the problem was gone afterwards. Interesting!