clunking - differential?

[jrobie79]

TL;DR - clunking noise, doesnt appear to be suspension, or any loose driveline bits (guibo, diff input/output flanges)

I have this weird clunk whenever I step on the gas coming from the rear. At first I thought it may have been my trailing arms as I have the weld on kit, so maybe they were getting loose...this doesnt appear to to be the case.

Whenever I shift from reverse to 1st I hear a clunk coming from the rear of my car. As I'm driving if I let off the gas peddle and coast for any distance and go back to step on the gas I hear the clunk. This makes me believe its in the drive line - loose giubo bolts, loose d/s to diff bolts, or loose axles, however it appears everything is tight. If I'm going downhill and put it in neutral and step on the breaks, nothing (which is why I'm suspecting the diff). My dad said something about it being inside the diff...worn gear or something that is causing some slop.

ps. I jacked my car up and drove it from 1st to 3rd up to about 40 mph and there was no noise, during shifts, letting off the gas and re-accelerating, or shifting into reverse/1st.

any help as its getting annoying and I don't want something catastrophic to happen. I'm going to swap my sisters open 4.10 in over the weekend and see if that helps. If it does turn out to be the diff, what is wrong with it and how can I fix it aside from buying a new diff. (fluid is full and less than 1 year old - motul synthetic)

if this is more appropriate in the driveline thread please move, couldn't nail it down the cause so I didn't post there

[VinniE30]

I had this problem and it ended up being the rear subframe bushings.

[jeffnhiscars]

I had very similar symptoms and it turned out to be my trans mounts had turned to mush. I know you said "rear end" but noises travel and a bouncing trans can cause contact elsewhere. GL

[jrobie79]

The tranny mounts are less than a year old..the subframe bushings are IE poly, also less than 1 year old. I'll take a look either way. Hopefully its the tranny mounts if anything - subrame is a bear.

[Quadrajet]

If you still suspect the Diff. when you have the car up, put the trans in Neutral and have the parking brake set. Or have the rear tires on ramps, then grasp the differential flange and twist to see if you have too much lash in the ring and pinion. If you remove the shields you can see if your universal joints are done too, while your there.

[SkiFree]

One more thing to check for would be the rear U-joint of the driveshaft. With the car in the air give the rear section of the driveshaft a good wiggle and see if there is any play in the joint.

EDIT: shoot, Quadrajet beat me to the punch.

[davem]

drive train lash, your dad is right.

[jrobie79]

drive train lash, your dad is right.

if it ends up being this, how is it fixed w/o buying a whole new diff? I'm hopefully swapping over the winter, so I guess I only need this to last till I put it away (first snow)

[davem]

just drive it nothing catastrophic will happen. this is just what happens with a manual car with this kind of drive train and lots of miles of wear.

[jrobie79]

I'm taking a trip to Miami the first week of December, I dont need my diff to explode on me in north carolina

[Vivek]

Check the locking nut on the drive shaft. It's a pain to get to, but that's what was wrong with my dad's car. Does yours clunk when you start off, ie as soon as the clutch hits the friction point and loads the driveline?

[Andy.B]

I had this problem, and after replacing bushing after bushing, found that rust had built up around the diff to subframe bolts around the bolt heads, causing the diff to be loose enough to clunk. Due to corrosion, the bolts felt tight. I had to use some heat to work them free, then re torque them down. Problem gone. Just something to check.


Sent from my Galaxy Nexus

[Holland]

This hasn't been mentioned, but check the shape of your axles as well. When run dry they will make a clunking sound when initially moving forward.

[eurotrash]

Also double check your subframe mounts. Poly is meant to be changed reasonably frequantly. Not a big deal on a race car as they dont see much driving aside from tracktime but dailying poly can lead to much quicker deteriation of the mount as they dont flex with movement like the rubber mounts would. Naturely your subframe being harder then the poly this leads to them breaking down quick.

[NLM Hollywood]

I've got this exact same problem and have yet to identify the issue. I do know that on one of the four E30's we built into race cars we had this issue and it was identified as slightly loose bolts from the driveshaft to diff. Since I'm not the one who found or fixed it, I'm not 100% certain, but it's something to check.