Una corda and dampers

[Victor Belanger]

Hello to all,
	I have a question regarding dampers. I have a "D" that I'm working
on, and would like to know if someone could explain this symptom. The
dampers work fine just the way they are, except when I engage the una
corda. There is now a delayed damping effect. Kind of like a "zzzing" after
effect.
	Could it be that the dampers have a particular shape, and with
vibrating only two strings it can't handle that? Is it time to change the
felts? Regulation wise, all seems ok. Any suggestion would be of great help.
I thank you in advance.
Regards.
Victor Belanger