Removed the doors on my 08 JKU to enjoy this fall weather and discovered a strange sound occurring between 45-60mph. Best way I can describe it is like shaking bolts in a can – it only occurs while on the throttle and happens precisely around 45mph every time (regardless of whether I am in 2nd, 3rd or 4th gear). It either goes away around 60mph or it becomes so quiet I can no longer hear it over the wind noise.

If I let off the throttle it goes away, but returns immediately as soon as you depress the pedal again - if I rev the engine in neutral, it does not occur. If I place it in 4-wheel drive the sound goes away instantly – returning to two-wheel drive will usually stay quiet for up to 15-20 seconds, then the noise will return.

I took it to my go-to shop and they are stumped so far. They suspected the rear driveshaft as a possibility (front replaced early this year with a CV driveshaft), but after looking at it they do not think it is causing the issue.

Any ideas? I am completely stumped on this one…

 

Amy ideas at all? Or at least the first couple of things you might explore if this was happening to you?

Been playing with it some more and it definitely goes completely away in 4wd, returning in 2wd, so I'm beginning to suspect the transfer case - but still don't understand what could be going on...

 

I am going thru the exact same thing right now. Haven't tried 4x4 to see if that fixes yet though. It is driving me crazy. Everytime I try to find info it says check welds on heatshield by crossmember. I put clamps on all the welds and it didn't help at all. I beat on the cats with a mallet to check for rattles and nothing, even data logged them to check for temp variances due to possible breakage, nothing. My sounds like a rod knocking when I take off on cold mornings, at operating temp it sounds just like what your saying

 

My next is to pull shields off exhaust manifolds to check for issues as the JKs are prone to cracking them. But the way my bolts look I just know there gonna break. You should be able to have other symptoms if it was your rear driveshaft. I have had one go out before and you can feel the bearings slipping in the CV joint

 

Did anyone ever figure this out?

 

Did the shop remove the shaft and check the joints? Not sure if you have a u-joint style rear shaft or not. If it is proportional to speed it sounds like rear driveshaft joints. Putting it in 4WD makes the drivetrain "share" the load required to move the Jeep down the road so less stress on the rear DS.
If it is the case then it shouldn't make the noises too long as it will fail perhaps. Did you do an oil check for both level and metal? But these cases are bulletproof as described by every auto technical writer unless we run em dry!

 

Did it sound like this?