Mira shower humming when heating-

We have a Mira Advance shower that was installed several years ago.
It has worked faultlessly until a week or so ago when the double pole switch began to make an electrical hum when the shower was on. It only made the noise when the shower was actually heating, running cold it was silent, it was defiantly the switch and not the shower making the noise. Anyway I changed the switch and all was fine.
Yesterday the shower itself started to make a noise, a sort of deep vibrating hum, again only when it is heating. Any ideas on what the fault is and how it can be fixed?
Any help much appreciated-
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