hum sound at 30 & 50

recently got a 2" bb on my 2001 cherokee sport 68,000miles
lift went on smooth, no vibes, steering firm and smooth, rides great,didn't even need aligned after, running 30x9.50 AT
It gave me 20in rear and front ,at center wheel cap to fender flare over my stock specs of 18"
my problem is when i coast and start to decelerate at 30 or 50 mph.
i get a quick hum/grr sound. the sound comes from under the back seat area/
the shop that put it on says it's not the lift or driveshaft, its my rear diff?
I never had a problem with the diffs or noise before the lift.

What do you guys think it is caused by? and how can i fix it?
i don't mind spending the money to fix it. i just don't want to ruin something by ignoring it. What would be a good fix for it?
thanks for any info!

if your 2" BB was the kind that uses a rear shackle, (not the add a leaf type), then check your rear exhaust pipe and if it is touching the shackle. If its close when the car is off, it will probably hit when the motor is on and its vibrating. I had the same problem on my 2000 when i installed the 2" Daystar BB. the fix most people do is 1: cut the tail pipe off so it doesn't hit (bad because exhaust fumes build under your undercarriage...especially if you do city driving with lots of idle time), 2: take a torch and bend the exhaust pipe away (be careful, you are next to the gas tank ... might wanna let a muffler shop do it). BTW, i brought my XJ to a local shop for some other service and when they drove it they felt this vibration at 40 mph and claimed is was my rear axle too... they spent hours trying to figure it out and when they finally called me i told them what it was. I knew about the vibration to begin with because when i installed the lift, the exhaust was so close i couldn't get the lower bolt through the shackle because there was no clearance. Good luck and hopefully this is your problem as its a easy fix.

My lift had ALL (long one)
i didn't have shackles

any guess as to what it might be causing it.?

Dont trust the shop, check your u joints.
When I did my BB I got what sounds like the same noise you discribe.
Turned out to be my rear joint. The shaft still felt solid but the joint was wasted .
Only started makeing the nosie AFTER the BB was on and you could tell it had been bad for some time.

Thanks for the advice drifto77

one more question for ya?
so after you replaced the u-joint you had no more noise?

so it wasn't bad pinnon angles etc.2" like some say, cause mine didn't change angles to much and i thought that wasn't it,
but i could see where the lift would put more stress on a bad ujoint,

so you'd say it was just a bad u-joint with more stress on it was your problem?

thanks! i'm checking mine out this week.
tony

TBCherokee said:

so after you replaced the u-joint you had no more noise?

Click to expand...

Right.

TBCherokee said:

so you'd say it was just a bad u-joint with more stress on it was your problem?

Click to expand...

Yes. Its been over a year since I replaced the u-jouint and the problem has not been back.
Like I said, the u-joint was bad when I did the BB but it was not noticed till the BB was on. Noticed it right away. Replaced both of them.

Hey just a thought - dont slow down at 30 or 50 . LOL