Central HEating?

My central heating system is old, and we have a tank in the cupboard for heating the water. On this tank is a pump, and when this pump is on, it seems to create a high pitch wail in the system. This is waking us up in the morning when the water comes on. We have bled all the radiators, but are not able to bleed the one in the bedroom as it has no valve (this seems to be where this noise is located) Is there anything I can do to stop this wail?