1999 Prerunner strange noise

1999 6 cylinder Prerunner with 261K. It runs great, I bought it new. I'm getting a random clunking noise from somewhere under the bed of the truck while going down the highway. I may hear it make the noise seconds apart or it maybe several seconds before I hear it. It's not real loud and no "lack of lubrication" noises, just a subtle clunking noise now and then. I have an idea of what it might be but I don't want bias anybody else's idea about what it might be. I'll say that so far I think it might be something to do with the driveshaft. Any ideas?

 

Hey, do you notice it changing at all depending on what speed youre at? Does it speed up and slow down in relation to your speed? Id start by checking the U joints and carrier bearing (if prerunners have a CB). Should be easy to check for play in them. If you have a spare tire under the bed, id check that too. My buddy had a clunking noise when he was driving and after a lot of searching, his spare tire was a little loose on the cable. It would clunk around under his truck.

 

I thought it might be the spare but it's tight and yeah, that's just the kind of sound I'm hearing, one that might be made by a loose spare. Well here's what I was thinking, I went under the truck and the shaft is tight and I lubricate the U joints. But the carrier bearing, that's what I'm suspicious of. It seems to still be lubricated enough but there is a lot of play between the rubber insulator and the center bearing so I thought the bearing might be banging against the rubber insulator now and then that surrounds it as the shaft rotates. I messed with the accelerator pedal a little bit earlier going down the highway but that didn't seem to make any difference one way or the other.

 

There shouldn't be any play in the carrier. Sounds like it’s no good.

 

I replaced the support bearing, it was still good, no cracks, still decent looking red grease in the bearing. I reconditioned the grease with synthetic oil and cleaned the rubber, covered it in Ballistol oil. I was impressed that it's in pretty good shape. I don't go off road much but I hit a lot of dirt roads. I'm glad I replaced it anyway, shot up the U joints with new grease. I still hear that knocking noise, it's not bad, curious about it and was wondering is something is about to fall off. Here's the old support bearing with 261,000 miles on it after I cleaned it up, going to keep it as a spare.

 

Next things you could check is drive shaft balance and rear end. You could have something in the rear axle thats loose/starting to fail. maybe check fluid for metal or check wheel bearings? Drive shaft out of balance isnt super common but if it is, it plays hell on your vibration and joints. you could try to jack up the drive tire in the rear with blocks in the front tires, and rotate the drive shaft when transmission is in neutral. The drive shaft should free spin without any binding. i just did the same thing with my front driveshaft and found out i needed a CV drive shaft.

 

Check your rear shock bushings.

 

The way it drives is perfect, as smooth as the day I bought it, no vibration at all. I turned everything when I replaced the center support bearing, I had the back wheels off the ground. Everything is smooth and tight. I really need to hit the highway to see if it's making that same noise. Imagine a nylon tie-down strap with the metal hook at one end hanging over the tailgate. Then the wind randomly blows it around and it hits the tailgate now and then. It's something like that. It's about that random also, bumps, acceleration, etc., those don't seem to change it much. I'll post if I ever find out what the cause is.