Accord Hybrid

2005, 54,000 miles.noise and vibration at 55mph. Sounds and feels like a straight drive in to high a gear. Also happens around 20 to 25 mph. Always when running in 3 cylinder mode. It stops immediately upon de-acceleration or acceleration, or manually shifting to D3 on the shifter.

Could this be a computer malfunction that tells the drive train to go to electric assist or to return to 6 cylinders?

Could be a motor mount problem.