92stangerang

after like 15-20 mph the car sounds like its going to fall apart. it almost sounds like your running over those safety bumps on the interstate. (wahh wahh wahh). i know its not the tires cause i just put wheels and tires all the way around and balanced. and its not the rearend cause i just swapped that out w/ 8.8. im thinking it might be my drive shaft. or the stock converter is fixing to go. im using a driveshaft out of an 04 gt. (it was long enough to complete the 351/c4 swap). is it possible that the shaft is out of balance? maybe bad u joint? its been doing it ever since i finished the swap. thanx.

87bird

if its the whole car it is probly the driveshaft.

Jfsram

The driveshaft would have to be REALLY out of balance to cause a vibration at 15mph.
Anything that obvious would be easy to see if you inpect the driveshaft.

Get it up on stands. Have someone rotate the rear tire and watch the driveshaft turn. Runout should be better than 0.20" but if it is the driveshaft it will be easy to see.

Not saying it's the tires but ruling out tire vibration just because they've been balanced is wrong. An out of round tire can easily pass as balanced. Again check for runout.

Stepman

Well, if it was the convertor, it would do it in neutral at the rpm's youo are turning at 15-20 mph. Driveshaft vibration most often shows up right at 35 mph. I would have to say move your tires front to back and see if the vibration changes or take them and have them checked again. It could also be a wheel bearing but, not as likely.

See if you can figure out if it's rpm related or mph related. Can you duplicate it with the car not moving.

ExtrmZ

is it when you get into (or out of) a certain gear? that seems when you would want to start shifting into 2nd... where does this sound come from?