I'm not certain that it's the pump. I'm not a boiler engineer myself. But when I feel the pipes below the boiler they're vibrating when there is the hum, which is why I think it's the pump. Also, the bathroom rad is sometimes slightly warm (even though the central heating is off), which also...
It's a combi boiler. The temperature gauge on the console also seems a bit high at times - 38, say, even if it's been idle for some time. I wonder whether perhaps it's overheating and the pump is drawing heat of the system?
I have a Worcester Bosch Greenstar 30 CDi boiler, about four years old.
I've noticed that it is giving out an almost constant hum. I think it's the circulating pump. This is even the case when, as now, the central heating has been turned off for the summer (I did this on the boiler console...