I have a Roland RD300-GX keyboard and a pair of Roland CakeWalk speakers. These are situated on the left and right of the keyboard, on a tiled floor.

When I play with headphones, the sound is crisp and clear, but using the floor speakers (not sure if they're called monitors), certain notes produce a very annoying resonating/distorting sound which totally ruins the sound of the piece.

Could anyone please suggest a way to remedy this?

Thanks in advance,