In the morning, when I start my car up and ease down my gravel street in 2nd gear, then slow at the stop sign, the rear end of my 93 SC400 makes a very loud THUNK noise somewhere toward the rear of the car. I was hoping my recent suspension replacement would solve the issue but I'm thinking it may actually be in the differential. It sounds like if you had a bowling ball sitting loose, then when the car slows to a stop the ball bangs forward - I can FEEL it as much as hear it - like some heavy part of the car is shifting forward with momentum as the car breaks. Strange thing is, this will happen maybe once or twice more at the next several stop signs, then I'm good for the day - no more. Next morning, same routine. Anyone have any ideas to throw at this?

 

Replace the fluid

https://www.lextreme.com/how-to-chan...-fluid-or-oil/

If its still bad consider buying another pumpkin or rebuilding yours

Check u joint

"Grab both ends of the joint and try to see if there is any play up and down or side to side. A brand new u joint wont flop around freely like a worn one does. If there is no play it is still worn somewhat and will be functional, it just won't have that new tight feeling. Once they develop play they should be replaced and will sometimes cause vibration if the play is bad enough." quoted from hmhatch

Non serviceable in these cars, could need a driveshaft?

 

Awesome - thanks a bunch - I'll give this a shot!

 

You may have a hard time diagnosing a bad u joint unless you take the drive shaft out. The u joint can be stiff and about the only way to feel it is flexing the cross back and forth with it out of the car. Your u joint probably isn't worn to the point of having slack, they almost never are to that point. But being stiff will make it throw out of it's center of rotation within the hanger bearing and thump at times or create pretty bad vibration. As documented on this forum in other places, these are not "serviceable" but can be replaced by a few driveshaft shops. I used this shop in the Atlanta area Drive Line Svc of N Atl, you send them the drive shaft they rebuild and balance it and send it back to you. Took about a week. Seems like it was ~$375 at the time, probably more now.

Most drive shaft shops won't take on a shaft that is a flanged/center pin connection on the end like our driveshafts are. I have never explored or asked why, must be the way you have to chuck up in a balance machine so you have to find a shop like the one hyperlinked above that can handle it.