I have read that this is a common problem, but I haven't gotten a solid answer on it, or if anyone else has experienced it here...
Vehicle: 2005 Honda Civic LX 4-door, 1.7L, 4-Speed Auto @145K Miles
Problem: Between 35-40 mph, the transmission will shift to 4th and engage the torque converter. The tach reads about 1500-1700 rpm after TC lockup. If light throttle is maintained, a significant, oscillating vibration can be felt from the passenger-front quadrant, near the drivetrain -- and is easily resolved if more throttle is added, or if you back off of it.
This is the only condition and speed at which it occurs and it does not matter about weather, A/C usage, etc. It is the lowest lockup engine speed in the transmission's operating range.
The best way to describe the vibration is a "flappy bumping" -- almost like driving on a washboarded road surface at that precise "wavelength", with that one tire. I would not describe it as lugging. The closest experience I had with it was when packed ice/snow caused a wheel to be out-of-balance and bounced the wheel/tire around -- but this feels more toward the the center of the vehicle by 1/3 (not center, not outboard).
I'm just curious -- it's a close family member's car, and I had just put a new alternator in it and was on the test ride. I thought "Oh @#$%! I've done it now!" When I came back, "Oh yeah, it's been doing that for about 8-9 months now". Whew!
Nevertheless, I'm curious about it. I'm trying to help keep this car on the road as long as possible.
RH77