My friend's car has 100,000 miles and when she is driving over 50 MPH, the car will gently vibrate a couple times every 15 minutes or so. It is not a hard vibration that shakes the dashboard, but you can feel it in your seat. I think it may be transmission related, and my first thought was the torque converter. She took it to a mechanic who thought the same thing. I was wondering if it could be something else? Maybe a bad torque converter seal?
The symptoms you described do make it sound like an issue with the torque converter. It sounds like, while you're driving, the torque converter lock up clutch is slipping occasionally, causing the vibration to happen. It is possible that there is a different problem that is causing the vibration, such as a torque converter seal allowing it to lose pressure. The car would need to be thoroughly diagnosed by a qualified professional, such as one available from YourMechanic, in order to determine what is causing your car's shaking issue.
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