Water tank sounds like it is constantly refilly every 5 seconds

[happinessstan]

This is begining to drive me insane! Water tank has been making a continual 'whooshing' refill noise every 5 seconds, 24/7. Upon closer inspection, it seems like the issue relates to one specific pipe/feed (on the left of the picture and not marked 'cold feed to tank') and can be heard most prominently in/behind the bathroom sink (potentially hot water pipe).
Can anyone help me to torubleshoot and resolve please? I have attached a picture of what appears to be the offending pipe and feed setup.
Thanks!

 

[Valveman]

Looks like some kind of pump? Maybe the check valve has failed. What water tank are you talking about?