Thumping underneath under deceleration

I've just noticed a new noise that is slightly alarming: a thumping underneath the floor under deceleration. It doesn't matter if I'm braking or just coasting, but I get this somewhat rapid thumping I can feel underneath my feet.

What's weird is that I removed my front driveshaft a week ago to diagnose another problem (which turned out not to have any effect), which is when I noticed the noise. I've started getting the "angry sparrows" problem on my rear junkyard driveshaft (with SYE), so I have a new Tom Woods unit in the mail to hopefully clear up any potential driveshaft-related issues.

Other than the transmission mount, I'm unsure what the culprit could be. Suggestions?

4.0, AW4, NP231, 141k miles

Trans mount, motor mounts, driveshaft u-joints, loose u-bolts, pinion and or carrier bearings in the rear end. Need to check all of these things. The angry sparrow noise as you call it tells me it's rotational. So that being said my guess is either a dead cardone joint or dry any dead u-joints on the rear driveshaft.

Thanks...i've had a hard time finding what is an "acceptable" amount of movement from the transmission mount...any insight? I can definitely grab the transfer case and wiggle the drivetrain around a little

My diff. pinion yoke has no play, so I assume that rules that out? My transfer case rear pinion has a very slight amount of play, which i've read is normal...

Trans mount inspection. I usually use a pry bar and lift. It's rubber so it will move/bounce around a little but usually not much by hand. If I see cracks I just replace it. It's cheap and easy. What about your u-joints in your driveshaft? Any back and forth(twist back and forth) or up and down movement of the u-joint in the caps them selves?

This SYE...


Is this a H&T?
Did you check to make sure the yoke isn't going to fall off?

You said you know your rear DS is bad (assuming it's a front used in the back). Is the slip joint OK? Any side to side slop? How about the Double Cardan, is the centering yoke completely shot? That could cause noise issues.

Finally, there should be no side to side or up/down or in/out action of the T-Case output.

magimerlin said:

What about your u-joints in your driveshaft? Any back and forth(twist back and forth) or up and down movement of the u-joint in the caps them selves?

Click to expand...

The leading ujoint to the tcase has tear in one of the cap seals (whatever you call those), so that is obviously junk. This shaft was a JY shaft from an early-mid 90's XJ, so at this point i was just ready to buy a new one

WB9YZU said:

This SYE...Is this a H&T?
Did you check to make sure the yoke isn't going to fall off?

You said you know your rear DS is bad (assuming it's a front used in the back). Is the slip joint OK? Any side to side slop? How about the Double Cardan, is the centering yoke completely shot? That could cause noise issues.

Finally, there should be no side to side or up/down or in/out action of the T-Case output.

Click to expand...

H&T, yes, and i have not yet checked it since i installed it years ago. I slathered the threads with Loctite so i would feel good about it staying put, but I should definitely recheck...

No play in the rear tcase output, correct? If so, i believe i'm good. My understanding was the front tcase output should have a very small amount of play, by nature of its design...if that's not the case i definitely need a new bearing on that yoke

A good way to judge the condition of the trans mount is looking at the exhaust pipe clearance over the crossmember.

If you cannot slide your hand easily, with extra room between the exhaust pipe and the crossmember, the trans mount has collapsed, and needs replacement. You would be quite surprise how much the drivetrain moves around on the factory mounts while driving.
The knocking noise you are hearing may be the exhaust pipe hitting the member.

I'll keep that in mind, thanks