Its a posh name for a matal frame with teeth, which the window motor drives and pushes the glass up and down.
 
Could be -P38 regulators are notorious for the teeth wearing out, usually it's the drivers or passenger windows as these tend to get the most use but the rears have been known to fail. Is the offending window travelling up & down freely or is it struggling??
 
If when the window is almost at the top it sounds like a machine gun, then it's probabley the regulator as the noise is the motor slipping on the ratchet teeth.
 

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