Need help diagnosing a weird noise.

[Radim]

I've been having a problem, if i go about 30+MPH i start getting a clunking, sound like something is loose underneath either trans, driveshaft when i let off the gas if im on the gas even a little bit it completely goes away, happens in gear or out of gear, it slows down as the truck slows down.

 

Just want to get to the bottom of it, don't want to have to take it to a shop where they will charge me an arm and a leg for something simple...(This is the first RWD car i've had so i'm not too knowledgeable on driveshafts ect..)

 

Thanks guys!

[Pumpkn210]

May be the exhaust hitting on something. Get under it and shake stuff till you find something. Pain in the arse, but we all gotta do it sooner or later!

[elmerfudpucker]

any vibration? checked your lugnuts? tranny mounts and motor mounts?

[hobbes_the_cat]

any vibration? checked your lugnuts? tranny mounts and motor mounts?

+1 also check and see if there is a lot of play around the front u-joint and front yoke where it fits in the tail shaft of the trans. buy always check the easy stuff first. ;)

[Radim]

Cool, i'll check everything you guys mention first thing tomorrow, i know lugnuts are tight though, trucks aligned, wheels are balanced too. Thanks!

 

Oh, it has a Dog-leg if that means anything aswell.

[metalmonkey47]

checked your lugnuts?

 

Ummmmm that's kinda important. lol

 

*Cough cough*

[elmerfudpucker]

Ummmmm that's kinda important. lol

 

*Cough cough*

 

Sorry to hijack, picture a sky jacked Bronco with no top sitting on 44" super swampers losing its front left at 40MPH. Front left disc hit the ground and barely kept control to stop it, meanwhile the tire bounced down the road and hit the hill on bounced up and into the back of the truck! Dude in front of us stopped got out and said " I CANT BELIEVE IT! I SAW THAT DAMN TIRE FALL OFF AND THEN LOAD ITSELF BACK UP!!!" I was a passenger and none too happy, damned thing nearly took my head off. :angry:

[banzai510(hainz)]

Is this truck really LOW.????????????could be driveline angle.

 

Put all 4 on Jacks put in gear and see whats loose

[datzenmike]

Check lug nuts, then drive shaft U joints for wear/damage or loose bolts.... there will be three of them. Next check the rubber isolator on the carrier bearing.... if rotted away the drive shaft will flop around. Now check the tranny rubber mount is secure by tugging on the tail stock trying to move it side to side and up/down. Now check the engine mounts.

[Radim]

Will do! Thanks!

 

The truck is lowered, it's on 4in blocks in the back, and height matched in the front.

[datzenmike]

Will do! Thanks!

 

The truck is lowered, it's on 4in blocks in the back, and height matched in the front.

 

4 inch! I think members with that much drop usually put a spacer under the carrier bearing bolt down to raise it slightly. Not my thing so maybe someone will clarify this.

[Radim]

Oh, sorry it was a typo, it's a 3" block in the back.

[datzenmike]

Oh, sorry it was a typo, it's a 3" block in the back.

 

Still enough to throw the drive line angle way out.

 

For the least vibration, the angle in should equal the angle out. Its hard to explain but when there is a bend at a U joint the drive shaft actually has to speed up and slow down as it turns. This is because it isn't turning in a circle any more but an ellipse or oval. Because the one end is being solidly turned by the motor and the other solidly locked to the pavement through the tires it is forced to turn at the same speed and vibration results. The bigger the bend the bigger the vibration. Now because there are two (or more) U joints, if the transmission to drive shaft bend angle is the same as the drive shaft to differential bend angle the U joints can be aligned to cancel each other out. Lowering or raising a vehicle can throw these critical angles out and cause vibration. Here is an example of how the drive shaft angles should be positioned:

 

 

.

Many thanks Beebani.

 

Shimming the tranny up or down or the carrier bearing (if equipped) or placing a small wedge under the leaf spring to change the differential tilt are all methods of correcting the angles. Indeed some blocks have a slight wedge built into them to correct for tilt changes. A worn U joint will also make this vibration problem even worse.

[Radim]

Thanks!

[Datsun fan]

and don't forget to let us know lol :cool:

[banzai510(hainz)]

this happen to datto510 on his 521 when he installed the 5 speed 5th gear would make a noise. then put a spacer above carrier bearing and was good.

 

They make a 3 inch 2deg angle lower block (Belltech 6202)

[Grey_Matter]

did the OP ever figure this out?

[Radim]

Yup! Ended up being bad U-Joints, thanks for the help and comments!