High speed "whump"

[packmule1911]

Got a rhythmic low pitched "whump" sound from the vehicle when accelerating at high speeds (passing someone) on the interstate. Seems to come from near the rear seat area, under the car. Let off the gas and it goes away, step on the gas and it comes back. It is not noticeable at lower speeds, but at 60mph and above its pretty prominent.

I'm thinking transmission mount. Rear drive shaft u-joint perhaps? What else would you folks consider?

Thanks!

[slvmart]

Doesn't sound like a U-Joint to me. But then again ....

I would check your tires, to make sure you don't have a problem with one of them. Like throw some thread, or even if you bumped a tire, that the cords didn't shift or get damaged.

[bcrosson92]

is it when the tranny downshifts? it could be the mount or u joint. crawl under and give things a wiggle

[packmule1911]

Noise when accelerating, and it stops or dies down a significant amount when I let off the pedal. Only hear it, and "feel" it a bit, at highway speeds.

[bcrosson92]

but its constant? not like it clunks when you put the pedal down and the transmission down shifts. sounds like a bearing or joint that spins. and the only things that spin like that are drive shafts/ diffs/ tcase/ tranny.

[packmule1911]

Sort of. It's like this:

Driving along at 60mph. Decide to go faster and step on the gas. I hear a whump-whump-whump sound with a consistent tempo. I left off the gas and it stops whumping. It's not a transmission clunk sound of feeling. It's a more dull, softer sound, like something out of balance, and being thrown out of balance under load.

That's why I was thinking transmission mount, or u-joint/drive shaft related. Under accelerating load perhaps a bad mount is allowing things to move around and get something out of alignment. When I reduce the load by letting off the gas, it stops.

Probably need to have somebody who knows something go and drive it, but that's the best I can describe it.

[bcrosson92]

actually could be motor mounts. the engine would shift back under hard power if your mounts are bad. if they havnt been replaced then they are bad. one is by the stock air box and the other is by the relay box on the pass. side of the bay.

[packmule1911]

if they havnt been replaced then they are bad.

Thanks! I've never replaced them. Had the Jeep since November. It wasn't a hard driver vehicle, so I'd expect that they weren't replaced.

I'll have to look into how tough it is to do. I have my garage, limited repair resources, and my knuckles need to heal up from last nights work.

[bcrosson92]

they aren't hard just use a long extension and a 1/2 drive

[packmule1911]

I read elsewhere that with a jack and block of wood you can lift the engine safely enough for the process. Is that the view here as well?

Which do folks like? DayStar are easiest for me to get, and afford. M.O.R.E. can be had locally for a good bit more $.

[bcrosson92]

yes that will work. if you cant afford an upgraded set id just get stock ones from a parts store. ironman andy makes a nice set and so does brown dog but they are more money

[sidewaysstarion]

I had a similar sounding issue in my rig, anything over 70mph you'd hear a rumbling from under the floor. I pulled the front driveshaft and the noise went away, I inspected it and the u-joints seemed fine, but after taking it to a driveline shop the issue was the slip yoke was worn out causing the vibrations.

[abebehrmann]

I had a similar sounding issue in my rig, anything over 70mph you'd hear a rumbling from under the floor. I pulled the front driveshaft and the noise went away, I inspected it and the u-joints seemed fine, but after taking it to a driveline shop the issue was the slip yoke was worn out causing the vibrations.

Do you know how to determine if the slip joint is worn out or not?

[sidewaysstarion]

Do you know how to determine if the slip joint is worn out or not?

Grab the driveshaft near the slip yoke and push and pull on it, if you can feel movement and see the slipyoke moving side to side you know it's worn out.

[4.3LXJ]

After all this, I would like to interject another thought. Since it only happens at high speed, how about taking a look at all the fender well liners, flares and other associated flexible wind dams on the vehicle. I bet one is flapping in the wind