If it isn't heating anything useful to you, you might as well turn it off to save fuel (i.e. if it isn't heating the DHW either).
It isn't dangerous because these things have interlocks on them that turn them off if they start to get too hot.
Sounds like a pump or a control valve to me.
my guess it's something to do with the pump or the electric control but no point in speculating if the engineer's coming tomorrow. I'd just switch it off to stop the noise.