I have a 2004 SLE with 30,000 miles new tires,that still has a vibration that I feel in the floor and seat not the steering wheel, it doesn't make noise and no change under braking. The 1992 Bonneville we still have also does this at highway speeds,it seems to me to come from the passenger side front, the tires I replaced because of age wore pretty even Any ideas? Thanks Dennis

both could possibly be a hub. lift one side of the front at a time and hold wheel at 12 and 6 and use a push pull technique to check for play. should be none.

also it'* possible that your front motor mount may be used up although i would think untimely given your mileage

Check all the tires, it might not be the one you think.

Roger.

I've had this problem on my 2002 and haven't found anything to make it better.

Same issue here...I have a list of other things I'm going to check...Front motor/transmission mount, wheel bearings, torque converter mounting nuts, harmonic balancer, driveshaft play...just got a front end alignment and wheels balanced...

Not trying to hijack your thread, but give you some ideas as I try to figure out a similar problem here. Checked my wheel bearings today, the passenger side is bad. Over 200K on the car and I don't know any history, so I'm just going to replace that one for now. Motor/Transmission mount seems questionable, I rocked the the car back and forth, the mount made some noise and there was quite a bit of movement. Going the cheapest route on this one, hockey pucks which I keep in stock for the bone chilling winters here in Indiana!

I have another thing to add to the check list, tierods and tierod ends. I didn't even get to the other things yet, lets just hope these are the only problems

Any updates?

Actually just took my car for a test drive and I still have some shaking at highway speeds. I replaced the wheel bearing yesterday and got it all back together a little while ago. I checked the wheel for play and I get a little bit of side to side motion.
I am thinking it has to be the tie rod end, I tried loosen its nut and the whole head pin started to spin, plus the rubber is cracking. The ball joint and the drive shafts appear to be fine and I really don't think its the harmonic balancer since it happens at random speeds and not specific rpms.
I am still planning to replace the front motor mount.

Mine starts at about 57 MPH and gets worse and worse the faster I go. Did tie rod end, wheel bearings are fine, and tried new tires.

Interesting....I haven't payed that close of attention, but I know mine doesn't shake going 50-55ish down the country roads. The torque converter should be locking @48mph. Had you said 47 I would say TC. I still want to check the TC nuts through the access panel.

I am starting to think it has to be the motor mounts...I feel the shaking in the body up front, not really in the steering...it will happen while speeding up, slowing down, or constant speed..