Not sure if it's related, but had a new mid-position valve fitted by British Gas and within a month a loud vibrating noise can now be heard from the mid position valve and cylinder area. This only occurs when the hot water supply is being heated up (1 hour am and 1 hour pm).
The noise appears to be coming from the valve when it is one particular position which may indicate the valve it's selve may be faulty. Again Guy I would refer this back to BG as you would expect some sort af warranty with this replacement. Good Luck with BG.