got a glowworm combi boiler under the stairs in a cupboard and when the heating is on u can hear a constant hum hum hum hum when standing on the stairs and can hear it faintly from boiler plummer came and had a look other daysaid it were coming from pump but everything working ok heating and water and thathe checked rads upsatirs for air and they ok so he said it could just b that its either weather cause its cold or just got noisy for some reason