Quickmill cracks loudly

Hello guys!
I have a quickmill 5000A for about half a year now I am also quite happy with it, but now it cracks several times quite loudly when the coffee goes through. In addition, the pressure drops slightly with every crack. Then I let a coffee down with the door open and noticed that the pestle gave way slightly upwards. Does anyone else know about this problem? And is there a solution? The machine is loud enough already without the crackling.
Lg Anita

Hello Anita,
my machine had the missing lubrication of the piston rod, which goes through this rectangular guide. I removed them, cleaned them and then greased them with coffee machine special grease (from Saeco). After that, the cracking was gone. Normal oil does not help because the piston rod gets warm and the oil dilutes too quickly...
greetings
Florian

Florian's right.