[AtlantaDan]

My 91 M5 has developed a really annoying (and expensive) sounding metallic knocking that comes from the back end of the car. I am inclined to think it's the LSD unit going south because it stops when turning and starts when moving forward or coasting. Normally, were it a CV, they usually click when cornering.... The sound is a cross between a tick and a clunk and can sometimes be felt through the seat. I know for certain it is coming from the back side of the car.

Fluid has been changed regularly with the proper stuff, but it's got 210,000 miles on it.

Thoughts?

 

[Blitzkrieg Bob]

My 91 M5 has developed a really annoying (and expensive) sounding metallic knocking that comes from the back end of the car. I am inclined to think it's the LSD unit going south because it stops when turning and starts when moving forward or coasting. Normally, were it a CV, they usually click when cornering.... The sound is a cross between a tick and a clunk and can sometimes be felt through the seat. I know for certain it is coming from the back side of the car.

Fluid has been changed regularly with the proper stuff, but it's got 210,000 miles on it.

Thoughts?

Diffs usually hum, whine, and roar. clunking and clicking could be a host of other stuff.... wheel bearing, bushings...The RTABs, diff carrier and the infamous rear subframe, CV (remember RWD... so they won't act up only on turns) Drive shaft center bearing and U joints.. Guibo?

Diff oil change? Did you put in 4oz of friction modifier additive? If not you could get a grunting on turns from that.

 

[OhneKompromiss]

Funny that Dan should have posted this, I've got a similar problem that began shortly after I changed the diff. oil in my car.

Bob, what is this "friction modifier additive" of which you speak? I've never heard of it, is it something you can just get at Autozone? Is there a brand you recommend?

Thanks in advance!

 

[AtlantaDan]

As an addendum - this knocking increases and decreases with speed and is rhythmic

 

[OhneKompromiss]

Dan,

This knocking, does it increase in severity and decrease in frequency when you are going slowly? In other words, does the severity decrease as the frequency increases as a result of vehicle speed?

When my driveshaft carrier bearing was going out, I had symptoms that were similar to yours. Basically, the rubber disc that "floats" the bearing within the bearing bracket had disintegrated, causing the driveshaft to knock against the tunnel when driving slowly. As speed would increase, so would the gyroscopic force of the spinning shaft, and it would center itself until I slowed down again.

Unfortunately, the bearing is pressed onto the shaft, making changing it out a pain.

Of course, all that info is useless if you've changed yours recently.

You can check the bearing by getting under the car and attempting to move the driveshaft. If it has more than about a 1/4" of play, or moves around freely, you may just have found your problem.

 

[AtlantaDan]

Thanks - I'll give that a check this weekend. Last night, I noticed that my CV's appear to be original and had little-to-no resistance when moving them towards the wheel and diff. Made me think that they were dry... The guibo was ok.

 

[Blitzkrieg Bob]

Bob, what is this "friction modifier additive" of which you speak? I've never heard of it, is it something you can just get at Autozone? Is there a brand you recommend?

Thanks in advance!

If you get a grunting sound when making tight turns, then that's it.

ATP was the brand I picked up, but Redline and a few offer it too.

Call around, usually only performance shops have it.