I have a problem I'm aost certain my drive shaft is knocking on the under side of the car just had the tranny rebuilt so I'm pretty sure that's not it I had the drive shaft balanced I went to install and still knocking any suggestions please!

 

Does it increase with speed, or is it only when you shift into drive or reverse?

 

Make sure and check to see if the bolts for the trans mount to trans are in
on both sides and the bolt or bolts that go through the brace to the mount
are in place as well.

 

"just had the tranny rebuilt"

Look first at the last thing "fixed"

or related parts

incorrect trans mount?

look for witness marks where something hit something else....

 

MAW check the torque converter bolts and motor to transmission bolts too while your under there.

 

No its ju when it's moving

 

There's marks where it's hitting the floor pan

 

Where the driveshaft is hitting? It has to be something you did with your repair, double check all your parts for correct installation.

 

Did you replace the trans mount? If so compair it with the old one to verify correct. Also did you take out the cross member? If so is it back in upside down? (If that is possible). Is it hitting the floor pan to one side or up at the top?

 

How about you tell us the year, model, engine and trans? What's an "Olds" 408?

 

Hmmm, not in the diag class of some of the above respected veterans but with your brief description, it could be as simple as needles missing from a U-joint cap to a more serious issue? Stuff a Go-Pro camera or phone under there and record that baby in motion.

Like Joe ^, detailed info is a must for the crystal ***** to focus.

 

I had a car do that once, shaft hit floor pan
so long ago I can't recall the cause/ cure.

motor mounts been replaced?

 

408......a 350 + 58 decibels of noise from underneath? 408.

 

I think the best thing to do is go under and see what it is. Yank on the driveshaft, visually look at the alignment etc.

 

At this point perhaps some pictures of your under side where its hitting, U joints, your transmission mount, and transmission cross member . Without seeing what your seeing we can only guess and have contributed many suggestions. I will add one more, worn body bushings allowing the body to drop.