Rattling noise from the valve thing

[OneTwo]

Could someone tell me what this is, please?
(coming from a Joule Cyclone unvented cylinder)

It was making a distinct knocking, vibrating, rattling sound, I turned it slightly clockwise, and the noise stopped instantly.

The system is 2.5 years old.

is there something I need to do to prevent it happening again?

 

[Best]

It is a bypass valve, the purpose of which is to open automatically, but only when needed to create a bypass. They can sometimes begin to create a noise. Best to get it checked out or possibly replaced as it needs to be at a fixed setting so to open only when system needs flow through it.
Actually best on a temporary fix to slightly open (anti-clockwise) rather than closing clockwise, so not to restrict or prevent flow.