Glo Worm Swiftflow 80, noise from boiler when heating off?

Glo Worm Swiftflow 80, when heating is off we get a constant humming/burring noise. Boiler is situated in the kitchen, but noise is worse in the bedrooms above. The engineer who fitted a new fan last October checked it out last week and claimed system was running fine! It's heating ok, but noise a night keeps waking us up. Also, when heating is off we don't always get hot water, sometimes have to put the heating on to get it. Any advice would be appreciated