Y freq 25hz -30hz - 35hz peak on VC4 IDEX 500

Hello, I don't know where else to search for removing my frequency : - rebuild both head - remove grease from linear rail and use 3-1 oil - check all zip tie - check the gantry alignment on the back... - tighten - un tighten all belts I still have the low peak and around 50hz Y peak are quite different between both heads. More over, MZV acceleration is lower then EI... They are supposed to decrease.. This is not normal. Have you any other idea? What worked on your's? Thanks