05 LX Sport Rumble at 50 to 65 Mph

I have a 05 LX Sport and was wondering if anyone has had any problems like the one I will describe below.
At 50 to 60 MPH I have a rumble/vibration that seems to come from the rear
of the car. I goes away over 65Mph It has been this way since I put the first 200 miles on the car (now have 6,800 miles). The dealer looked at it a couple of times and it has been duplicated on a lift. The dealer wants me to put more miles on it to "see if it goes away or gets worse".
Anyone else with a LX Sport noticed this problem? Could it be the new linked rear suspension system and a resonatation issue?
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wrote:

Ask them to spin balance all the wheels/tires. I had a brand new car a couple of years ago that had the exact same symptoms. After they balanced the wheels again, it was perfect.
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Tires and wheels not the problem...
the car is as smooth as silk.... Just getting a very low frequency sound from somewhere under the car between 50 and 60 MPH

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wrote:

Sounds result from vibrations. Vibrations can be caused from improperly balanced wheels. You cannot tell, by looking, if they are not properly balanced. You need to put them up on a balancer. Don't assume it's correct just because it's new. Mine was factory fresh new and it had this problem.
If it's under warranty - have them do it. You have nothing to lose.
You cannot rule it out until it's done.

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