TECHNOZEN is spot on.
This could be the usual fault with the ZS alarm, dampness inside the sounder. The sounder is located under the water expansion tank, remove the expansion tank, disconnect the sounder, take it apart, if there is dry white deposit inside it's got water ingress, dry and clean the inside, re-seal and try again.
In my experience the solution is to get a replacement. I got one from a breakers for £25, bargain!
Hope this helps.