I practice mostly with headphones as I find the piano sounds better to me while keeping the volume at very quiet levels.
When I do play thought the built in sound system I sometimes hear a vibration, an extra noise coming from something vibrating sympathetically. It took no time to find out where the sound came from. (I recorded myself playing the specific music that created the vibration and was then free to locate the source of vibration using the playback memory.)
Under the piano there's a piece of plastic housing that's about 18" x 5" x 1.5". This covers the two transducers that provide the TRS \Tactile Response System\ 'string' vibration in the key bed, keys and pedal; the plastic housing also serves to hide some cables.
If you slap that housing with the back of your hand you'll hear a hollow sound that's just asking to be damped.
There are about 10 screws holding it place of which three are recessed. I removed the housing and placed tubular compressible foam around those three recessed thimble-like plastic fingers that those recessed screws went through. (It's a lot easier to see this than to describe it.) The foam I used was longer than the three recessed points which then provides constant pressure against the housing. (I could have also just left the housing permanently off as you wouldn't see the cables unless you were on the floor.) I also wedged another piece of foam between the housing and the piano.
So far, so good. I'm sure an engineer could have come up with a different solution, but this seemed easy to me and fortunately I had the perfect tubular foam for the job.
One more thing, for those here who also own an N3, that plastic housing doesn't just drop off when all the screws are removed, I had to pop it off by inserting a screwdriver under the edge and forcing it to move slightly.
