High frequency vibration front end
 

About 8 plus cycles per second vibration (very rapid) heard and felt in front. Now louder than previously. thought it was right front wheel but all tires and suspension check okay. Noise seems to be slightly right of centre of engine area, not as far over as right wheel. Occurs at slower speeds and lasts briefly; it will stop immediately I relax throttle pressure.
Could this be tranny or engine mounts?
2001 civic with 266000kms.

How are the fluid levels in the tranny?
Engine oil level good?

Check the front axles and the motor mounts.

MOre clues: it happens at 1600 rpm only in overdrive (D) and seems to happen just when it shifts into overdrive (top gear).
The noise is just like the noise made by the shocks when driving over railway tracks, but is not that. Noise comes from almost the centre of the engine, as heard from the cabin, not the wheel.