Family member appears to have severe water hammer in high pressure
Santon HW system. The offending item (where the vibration is
concentrated) is the three way valve thingy (about 150 - 200mm wide).
It never happens/can't be reproduce when the installer comes back but
I've heard it first hand and felt the vibartion in that unit.

Pressurised HW systems a bit of a black art. Any ideas what's wrong
and how it can be fixed.

Naffer