The more I think about it the more I believe that DM was using that pin 8 I mentioned for disabling the amp when not driving the spindle. I could be wrong, but it makes sense to me. Before I removed all the electronics I played around with the machine quite a bit, and not once did I hear that sound. Actually, I did hear it, during a tool change, when the spindle was turning very slowly, slow enough to hear it. But not any other time.
If the motor/amp combo was being used for an axis it would make sense to leave the amp enabled even when not commanding an axis move, in order to keep it in place. But being used for a spindle motor, when the spindle is not running, you want to be able to turn the spindle by hand for tramming parts (well, us old time machinists do anyway, I know there's other ways like using an edge finder, etc.) and with the amp enabled it's not possible. When I jumpered pin 8 to 9 (ground) it cut the amp along with the noise and allowed me to turn the spindle. It's not a permanent disable either, that is, it doesn't require a reset, so once the ground is removed the amp is re-enabled automatically. I think this is what I'm going to shoot for, using an output on the parallel port (tied to the 'spindle on' command) to ground the pin until commanding the spindle. I'm thinking I'll need a small relay for that, not sure.