If you take the route mentioned be very careful. Those keys are weighted and balanced to Roland's specs out of the box. Adding felt to the top could hinder the key return a bit, and possibly cause other sound, and hardware related problems.
I do however understand what you mean about the clank. Some keyboards are louder, where as others produce less noise. Again, just be careful with your modification.
GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.