strange bump sound

[jag7720]

When I brake and come to a stop, sometimes, when I accelerate, I get this bump, more of a bumm sound. It seems to be coming from the rear. I don't even know where/ how to start trouble shooting that.

It is not a clunk or a thud... the only way I can describe it is a soft bumm and it doesn't happen every time.

I have new u-joints and a new yoke in my drive shaft.

Anyone have any ideas

 

[Barrman]

Crawl underneath and see if you can bench press the transmission up a few inches. My first thoughts are transmission mount or worn leaf spring bushings. Maybe shock mounts too, but they tend to rattle.

 

[flippydidit]

I'm with Barrman on this. The first step is 'close with, and destroy'. If the strange noise is coming from behind and underneath the truck, then the first step is to get down there and look. Could be something simple too; did you check to make sure the spare tire is secured, or maybe something in the bed? No way to really know until you do some "inspectigatin'" .

-Keep 'em shooting

Nate
Army Infantry 9 1/2 yrs. '97-'06
Gunsmith/Government Weapons Contractor
1985 M1008A1 (slightly modified)

 

[jag7720]

OK, it has been a while, but I have since had the trans rebuilt and the shop said the tranny mount was fine.

I am still getting the "bump" although it is lighter now.

Everything in the truck is tied down and secure. It is definitely coming from under the truck.


Any ideas what is going on?

 

[Nick Danger]

Does this happen no matter the fuel level ? sometimes fuel slosh in the tank will give a slight thump

 

[EZFEED]

Check your U-bolts on the axle, check to see if there is any slop in the driveshaft U-joints and the input yoke on the axle, also check your spring hangers. Loose bolts on these will cause the whole axle and spring assemble to shift back and forth.

 

[Nick Danger]

Try some grease in the drive shaft yoke it could be binding in the transmission

 

[ohnuts]

Tape a baby monitor around in grid locations under your truck. Graph the locations and the volume. Send the recording to Seti, wait for the responce.

Or maybe not.

 

[jag7720]

ok, this is old but it is still happening and I really want to figure it out.

Anyone have any ideas about this?

Rebuild tranny
New U-joints
New gear lube in front/rear diffs - When opened I didn't see any signs of damage...


One thing i noticed is that if I come to a stop and then lift off the brakes and let the truck roll a bit, the "bump" doesn't happen.

 

[niferous]

I would get under the truck and index the drive shaft and then pull the four bolts that hold it on off. Then check the u-joint's side to side and up and down movement. If it's not smooth you should go ahead and replace the u-joint. If you don't have a good shop vise you will probably have a hard time taking out the u-joint and putting the new one in. Most drive line shops can replace them for very cheap though. If you decide to do that route all you should need is a white paint marker and a 7/16 in wrench. Just draw a line from the drive shaft to the rear dif and do the same to from the driveshaft to the rear of the transfer case. Then just remove the front four bolts and the rear four. You might as well have them check the balance on the shaft while it's there as well. May smooth out your ride a little bit.

 

[jag7720]

Thanks but as I mentioned, the u-joints are new. The tranny shop replaced them when i had the tranny rebuilt.

 

[niferous]

Sorry I must have skipped over it by accident. Eager to help I guess! Does the bump happen before or after the trans shifts? If it happens before or after is it uniform in time to the trans shifting or does it just happen whenever?

 

[jag7720]

it happens before shifting... usually in the first 10 to 20 feet... unless I let it roll first before pushing the accelerator.

 

[niferous]

Does it seem to lunge forward a little when it does this?

 

[jag7720]

Nope... just this soft bumm

 

[niferous]

Honestly it sounds like something in your tranny just shifting. You may take it back to the shop and have them look at it. I had something similar in an old Chevy truck.

 

[lavarok]

This is an interesting thread. I notice a similar sound in both my M1009s. Not very often but I know the sound you are referring to. Its not very loud, but ominous none the less. One day I will crawl under and inspect all the items mentioned here. Up to now, I have attributed it to normal old truck noise!

Which model CUCV do you have?

 

[dogkiller]

grease?

it sounds goofy, but: gm has had problems with the sound you describe,
I have seen it on at least 4 trucks pull the yoke out of the trans & clean & grease the splines...
on mine the sound would happen when you leave a stop, not while shifting..
good luck..

 

[wallew]

I hate to ask the obvious question, but which CUCV are we discussing. I have found that to be vital info.

Sorry if I missed where you posted that info.

 

[mikes47jeep]

turn up the radio, the sound will disappear


just because the tranny shop put new u joints in dosnet mean they remembered to grease them

i saw that happen a couple times

also i could be your rear brakes sticking a bit

are you sure your spare or jack isnt moving around?

is your tailgate moving slightly?

are any of your seats loose?

there are so many things that could make a bump