You could try removing the trv heads so that they wont affect the bodies at all. See what happens - ANSWER - we have removed all the TRV it's made no difference.
Do you have your heating on all day and night? If it's not on during the night but it still makes the noise, i'm not sure how trv's can cause this. Is the boiler preheat on? - ANSWER - When it's cold the heating is on for about an hour in the morning and 3/4 hours in the evening, but even when the heating hasn't been on at all, it can suddenly start making the dull thudding noise for no reason! Never had a problem when the boiler was first installed, it was OK until winter, when we started to use the heating, that's when the noise first started. Sorry don't know what you mean by boiler prheat
Was this the guy who installed it - ANSWER - No this was an engineer from BOSCH who said he couldn't find any fault with the boiler.
Check your expansion vessal , pipe to the vessal could be blocked , had the same problem on a boiler before, ANSWER - will try that, thanks.