I take an interstate for a short commute to work. This morning after about 5 mins at highway speeds (~70 mph), the car starts to vibrate. It feels like maybe 5-10 Hz...kinda like wheel rotation speed maybe? A rapid, light thumping. Not jarring, but quite noticeable. When I got off the interstate it went away. When I drove to lunch later, speeds lower than 60, interspersed with stoplights, no problem. On my way home, again, smooth sailing for the first few minutes of the drive, then the vibration returned. Upon reaching a stoplight near my home, the remainder of the drive was low speed...no vibration. That's as much testing as I've been able to do so far. Tire pressures are okay, wheel lug nuts are okay.

Any ideas?

 

If the vibration goes away under decel you most likely have pitted inner CV joint cups. There is a FAQ DIY repair

 

https://www.s2ki.com/forums/index.php?showtopic=241367

 

Get your wheels balanced.

 

could be a alignment issue or wheel issue, possibly a bad tire

 

If the vibration goes away under what?

Thanks for the feedback.

Edit: This occurs at steady-state speed, not under acceleration, if that makes a difference.

 

Sorry, under deceleration. Go closed throttle/coasting and see if the vibration changes.

 

Slows2k is right....
The CV bucket swap fixed this same issue with mine.

 

To test, get up to speed and wait for vibration to begin, clutch in to nuetral and see if the vibration goes away. If it doesn't - its most likeley wheel weight related. If it does go away, then it is quite possible the CV cup has pitted badly and needs swapping or replacing but there is another way to test for that specificaly - find a couple of SHARP left and right hand 30+mph corners and go through it in 2nd or 3rd at WOT- if its the CV cup the vibration will be noticable on the side with the most load - you have to get the revs up to at least 4K and go WOT to feel it. If you've never replaced/swapped them I would bet money its the drivers side as that one tends to go first.

If that doesnt work, jack her up and pull on all four wheels by hand to check for a wheel bearing starting to go.

 

Tested again tonight. Definitely still a vibration even when decelerating with foot off the throttle. Below about 50 the vibration is not noticeable. I didn't think to put the clutch in while on the highway, unfortunately. It feels like a whole-car phenomenon...I can feel it in the steering wheel, but not to the degree I would expect if there were something wrong with the front wheels. I gotta think there's something wrong with one of the rear wheels/tires, though the few-minute delay between reaching highway speed and beginning of vibration is confusing me.