Stanton premier unvented hot water system

[JC82]

Hi folks, Im have an issue with our hot water tank. At random times there is a really loud clunk/bang coming from the water tank upstairs, its hard to put any pattern to this. Its arguably got worse over the summer but this has been going on for about 10 months. I'm guessing its possibly a valve or something not doing its job.
Any ideas would be appreciated??

Thanks in advance

 

[ShaunCorbs]

Best to get a G3 registered engy out as it's an unvented system