Heres the story:
I put the Vexta steppers in my MKI - speed increased stock
Installed 3/8 - 12 leadscrew on the Z axis
Installed belt mode on the Y axis.
Unit ran fine for 3 months or so, then............. The Y axis started chattering while gantry was moving.
Tore unit apart and cleaned out the gunk that had built up in the bushings - chatter still there
Decided to go ahead and do the overhead mod to keep the foam dust out of the bushings.
Installed new bushings into the Y axis and got my linear rods installed - smooth as butter.
Installed belt and got the belt set up perfectly, moved the Y axis by hand..............Chatter, WTF????????
took off belt and chatter was gone.
Installed basic stepper that came with my MKI that I had kept for emergencies. Chatter gone.

So, if you hear chatter coming from your PP don't rule out that the steppers can cause this.
BTW, the chatter happened on bad stepper with or without being powered up.

Off to order a new stepper from Mark

 

Can I assume you checked the pulley and it was tight on the shaft of the motor?

 

I had to tighten my belt quite a bit from my first adjustment to get rid of the chatter.

 

Belt and pulley were tight grub screw was firmly tight on the flat spot of the stepper

 

Tighter!

 

For safety reasons I tighten as tight as it will go. And then give it one full turn. Your results may vary.