Thumping noise ?

[Brian VT]

Sorry. I'm a newbie. I searched some but came up empty.
'96 XLT 4.0
It makes a FOOMPF sound.
The best way I can describe it is imagine you had a 3/8" rope tied to the underside of the car and drove off. When you reach the end of the rope it would snap, making the noise and giving the car a slight jolt.
It kinda seems like a misfire (wife's theory) but my mechanic says no. At lower speeds I can sometimes detect a slightly metalic note to the sound so maybe I've got a drivetrain issue but that tone could just be from a loose heat shield ?
It happens randomly. I'm not able to make it happen. Sometimes not for 10 minutes. More often (less than 30 seconds) on slight inclines. Never when hard on the gas. Never when coasting. It happens at steady speeds, going straight or turning, on flat or uphill smooth roads.
It has the switch for AUTO,4WD,4LOW. No 2WD. I've run it in all modes and even driven in reverse in case that is still needed to release the hub locks ? Tranny oil and filter were changed 10K miles ago.
Tonight I did the brown wire mod thinking that maybe the transfer case clutch was the culprit. Nope, but I'm happy to be rid of the Auto function !
Any/all theorys appreciated. My mechanic says his best guess is a new transfer case, but it'd be a $1500 guess.
Any/all opinions welcome.

 

[skoal_mint]

sounds like somthing may be binding...maybe your cv joints?

 

[Brian VT]

sounds like somthing may be binding...maybe your cv joints?

I like that idea better than the TC. Would the sound be coming from the front ? My sound is near the center of the vehicle.
Thanks for your time.

 

[Brian VT]

I'd been trying to associate the noise with a particular situation.
Just tonight I noticed it seemed to do it just as the suspension was "unweighting" i.e. after a small rise.
I can't say that this is the only time it does it, but it seemed to be common timing tonight.
U-joints ? Shaft splines ? Rear end ? CV joint ? Transfer case ?
When I joined and posted this I figured I'd get flamed for not finding my problem through a search and be pointed to a bazillion threads on the subject.

 

[biggs92EB]

my ex did something similar to that but it was more like a "lak lak lak" and then it would stop and then come back later...turned out to be the little keeper bolt that holds the pin that holds the spider gears in had broke and the pin was slipping out of position and hitting my pinion gear. its an easy check, and hell youd be changing your rear diff oil while your there if that wasnt the prob

 

[Brian VT]

Muffler bearing ? lol
Still stumped and it's getting progressively worse. I've got a 200 mile trip ea. way this weekend and am nervous that something will give. I guess then I'll find out what the problem is.

 

[BlusploderAWD97]

Yeah, mines doing it to. But I can make mine do it. Whenever I accelerate quickly or any time the rear tires would slip it does it. I think I did the wire mod right, but it still does it. I've isolated it to either the transfer case ot the frtont left CV joint. O hope its not the whole axle.

 

[Brian VT]

Muffler bearing ? lol
Still stumped and it's getting progressively worse. I've got a 200 mile trip ea. way this weekend and am nervous that something will give. I guess then I'll find out what the problem is.

Sorry for not posting back sooner.
It did let go...100 miles into my 200 mile drive. I had my dog with me and my dirtbike on a hitch rack and it was 5:30 p.m. on a Friday night. Amazingly, my mechanic came and got me with the flat bed.
It was the transfer case. $800 for a used one, installed. I hitched a ride with a buddy to the race so all was not lost.