Done all that I know to do:

-installed new spring;
-decreased tension on old armature spring;
-straightened switch leafs and adjusted for minimal tension;
-switched armature with no wear from a non-hold relay;
-measured resistance of coil, 22.3 ohms
(Pinrepair gives a 9740 coil 24 ohms);
-all 9740 coils measured 22+/-;
-set transformer back to normal tap.

Are 2 ohms less resistence enough to be the cause?

Going to find a 9740 coil from another game that meets the 24 ohm resistence and see if that changes anything.

Anything I missed? Thanks.

Dan

Thanks to Flippy on RGP. He suggested checking for the copper washer at the coil screw. It was missing.

Posting this for future reference. Installing the washer quieted the buzz and it required tweaking the switch contacts.