Strange Vibration. not the shimmy please help me pinpoint

[Jason]

At 65-70 mph the car will occaisionally vibrate. It feels like if you were to accelerate from 20 mph in 4th gear, perhaps not as prounounced. It is not dependent on engine speed or gear. The guibo has been replaced and it doesnt feel like that vibration. I can't pinpoint it. It doesn't shimmy and its not coming from the front of the car though. Nothing changes when it happens, temp, fuel consumption etc all stay the same. I notice it when i cruise between 65-70 but it doesn't happen all of the time. If i coast down to 60 it goes away and operates very smoothly just like normal. If i speed back up it may or may not back. Tranny mounts are ok, flex disk and all suspension componets are ok, brakes are fine, the wheels are properly balanced. the only thing is i have a slow leak in my left rear tire, but i always check the pressure and make sure its full. The egine idles smoothly and pulls fine and makes no noises at any RPM even during the vibration. It feels like a drive train issue. Could it be an out of balance drive shaft? if so how would that happen? As usual, any thoughts are appreciated.
Regards,
J

[632 Regal]

typicar signature of an out of round tire...been there done that. when it goes away it's cause another tire that isnt round gets the proper phase and cancels the first offender.

Have fun with it!

[DanDombrowski]

First thing to try is to take it out of gear at 70 when its vibrating. If it stops, its a drivetrain issue, but I doubt that's it.

When you say that its like accelerating at 20mph in 4th gear, are you saying that its a higher frequency vibration than a shimmy or out of balance wheel?

[Jason]

not at all like the shimmy. not like the guibo either. like a cell phone on vibrate but on a much larger scale of course.

[632 Regal]

did it start after you did the guibo? Check the half shafts and cv joints, something has to be visably wrong to cause that.

[Jason]

"thud thud thud thud thud" feel... i dont know, its a first for me

[Jason]

This first reared its ugly head about a month ago and has come and gone. I'm leaning towards the tire now though. I haven't been paying attention as to whether it corresponds to its PSI which seems like the main variable im overlooking.

[DanDombrowski]

if its a higher frequency than the angular velocity of the wheel, its either a driveshaft vibration or a self excited vibration (due to rubbing) of a wheel bearing or other moving component. Check to see if it goes away when you take it out of gear and go from there.

[Jason]

anything i can inspect that would show me rubbing?

[Bill R.]

Like George... hope he's having fun wherever he is.. Anyway all the big words aside, a driveshaft vibration rarely rears its ugly head above the 30 to 50mph range..Its most likely a internally separated tire which would account for why you don't feel it all the time..I would run my hand around the tread surface of each tire and see if you feel a bulge anywhere...Frequently this type of separation feels worse when the tire is hot such as a warm day on the highway with blacktop at higher speeds. When its cold and the tire hasn't warmed up you don't notice it as much.. If this is the problem then that tire is going to come apart on you soon..

if its a higher frequency than the angular velocity of the wheel, its either a driveshaft vibration or a self excited vibration (due to rubbing) of a wheel bearing or other moving component. Check to see if it goes away when you take it out of gear and go from there.