Strange sound when printing y-as slipped?

Anyone recognize the sound when moving the printer head?
I thought that it was a lose pulley so i took the stepper motor apart and checked it. It was still tight and no movement detected.

I marked all the pulleys and rods with a marker so its easy to see if any pulley is slipping.
The tension of the belt is the same as the x-as and its not to tight and not to lose.

After this dubbel check i’ll printed the object again. Still the same sound en slipping y-as.
I thought it might be the stepper so i swapped it with the stepper motor from the second extruder.
I’ll even swapped the driver board but no succes.

The same thing happened with 2 other pieces when printing. We also managed to print correct objects

Does anyone has any clue what the problem is?

Included:
Video movement + sound
http://youtu.be/FJoPCpULy1A

hmm this does indeed sound strange…

Can you post the gcode you used?

Can you check the travel speed in your slicer?

IMHO it’s a bad axis alignment : when moving slowly, no problem. When the head starts a quick move it gets blocked and the noise is the belt slipping on the motor pulley.