Probably the safety valve is leaking - it may have been overpressured and vented and now has a bit of dirt in it, ot it may have failed.
"Triton Aquatronic" covers a range of shower heaters so the exact part will depend on what you have.
This site:
https://www.showerdoc.com/shower-spares/triton
will be able to help and has a section for questions as well. If no one else has had your problem and there is no solution posted already, ask away.
ACP, August 2017
