Squealing z motors

The mm per steps is set for 3200, which was working last week. Now at this level the z motors squeal and do not move. Attaching any other motors to the Z circuit; X, Y, or an extra motor; causes the squealing. Setting the mm/steps at 320 there is no squeal, as I approach 1000 the squeal starts.

What's the feed rate?

One thing is how many steps take to move one mm and another quite different is how fast you wanna do it.

Have you set the current correctly in the stepper driver? To much or too little and the steppers will malfunction.

Your steps per mm shouldn't be a guess, and 320 sounds like the microstepping isn't setup correctly. Your steps pe rmm should be calculated based off of the motor step angle, the microstepping, and the screw pitch

Unless you've got wierd motors, they're gonna be Nema 17, with 1.8 degree step angle
If you're using a RAMPS based board, make sure you've got all 3 jumpers placed under the Z Axis Stepper Driver, that'll be 1/16 microstepping

Last thing in question would be the size of the screw

Knowing that, use the Prusa Calculator to determine your steps per mm
And put that in your configuration file

Also make sure the feed rate isn't too high,