While driving my 03 MN6 coupe I get a pulsating vibration in the car between 78-82 mph. it happens only at these speeds and in 5th or 6th gear, the vibration comes in 1 second bursts and happens about every 2-3 seconds. the car has 9500 miles on it and none of the mods Ive done would affect or cause this problem. any input is greatly appreciated

 

Where exactly is the vibration coming from??

 

I am willing to bet you have a bent rim. The runflats dont give much and when you hit a pothole or bump the rim can bend. I know I bent 2 rims in the first year. I got rid of the runflats and havent bent a rim in 3 years. The wagon wheels were notorious for this but the later model wheels should have been better.

 

you might have 2 wheels slighly out of balance, probably 1 front and 1 rear. when the imbalances get in phase, the vibration is magnified. have you have the tire balance checked?

 

sounds like a wheel problem, youmay have even lost a wheel wieght

 

My money is on the wheel balance sync thing. Have the tires spin balanced.

 

Since you can duplicate the problem in either 5th or 6th gear at the SAME speed, your drivetrain is out of the picture buddy; got to be the wheel(s) and/or tire(s), as suggested.
My concern is you shouldn't be able to bend a rim with the tire; I'd try to get your problem warranted first and see what happens, in case you don't have any evidence of damage to your wheels. If you have the least evidence of your wheel contacting a hard surface (pot hole, curb, etc), you're on your own on that one. Same if it turns out to be only a balance job; they'll say it's a road hazard.
My car vibrates at certain high speeds (don't remember exactly), but I picked up the car at 8pm, and took off home very early in the morning. After 700 miles to get home, I knew dealer would reject my re-balance request, so didn't bother to try. I won't re-balance unless it gets worse, mostly because my wheels will look like poopie when I remove the factory weights; why in the world does GM have to use weights that damage $2K wheels? Does paying $1,250 extra for the polished wheels don't include $2 stick-on weights? Geez; I'd have paid $2 extra for them.

JC

 

Ok fellas ill give the wheel balace thing a try- I did hit a pothole just before this started but I figured no bent rim, no problem. THANKS Guys!