The spill valve is mounted on a removable board, and the valve should
have a thick punching of green action cloth (similar to back rail cloth
for a keyboard-style piano) attached to its inside surface.

Remove the board from the reservoir and see if the punching is missing.
If it's still there, see if it's a little smaller outside diameter than
the diameter of the hole in the mounting board, which will permit the
vibration.

In either case, replace it with the same thick action cloth cut to
exactly the size of the hole, or a tiny amount oversize, so it drags
a little on the edge of the hole all the way around. Make sure it isn't
too big, or the spring won't be able to close it all the way.

With a slightly tight fit, the friction should stop it from resonating.

Art Reblitz - Reblitz Restorations Inc.
Colorado Springs, Colorado