We have a similar boiler to yours and we had a lot of trouble with it. It also made a humming noise but it also used to make a loud banging noise - especially if the hot water was being used. After much investigation we were told that our sealed, pressurised system must have a tiny leak or seepage somewhere. We never found a leak - I suspect it must have been under the concrete floors - but eventually the plumber poured sealant into the system and pumped it round. After about 2 hours the system sealed itself and we have never had any noise since.
It may be worth asking a plumber about yours.