I have a 68 Mustang 200 CID automatic
I have a vibration that I feel in the floor pan, that is only felt while under power. I looked under the car and found nothing loose.
Any ideas what could be causing this?
If the driveshaft is out of balance the vibration is speed sensitive. However, if the U-joints are out of phase(running a different angles front and rear) the frequency will be speed sensitive while the amplitude will be power sensitive. A change in the differential orientation (lower for instance) affects phase.
Your driveshaft could be out of balance. Get it looked it. If it breaks under load, you and your vehicle can receive serious damage.
Look get under the car near the end of the trans: put you hand I the drive line try to push up-and-down and right and left . Then go to the rear end and put your hand on the driveline and do the same thing up and down and right and left : checking for dad bearings. I had a 67 v6