MC-4VA Servo Timing Board TB2 OSP 5000

We are having a rumbling noise in all 3 axis of our mill. There is a mild to moderate rumbling vibration that can be felt in the machine while it is sitting still. Like the axis servo motors are fighting to hold position. The noise is worse while any of them are moving and it is affecting part finishes and sizes, obviously. There are times when the noise goes away and the machine runs as it should, but those times are getting less and less.

The control is a OSP5000. We have tried replacing the 2 different SVP boards with no difference. The next board we want to try is the Timing Board 2. Ours is labeled TB2 (E4809-770-011) but I am finding a lot of -A and -B versions. Does anyone know if these are interchangeable?

Thanks for any help.