High frequency PE error offen caused by the eccentricity of the pulley for the belt or gears.
Additionaly It also caused by the precession of the worm screw.
There are many mechanical causes of PE errors.
If you can download this paper(Sorry this paper written in Japanaese, Famous mount maker's report about PE error)
Please look at the pictures of this document. You can imagine about PE error.
you should try to record the guide log and check the details in a PHD log viewer etc.