horn-like noise

I have a baxi combi-boiler. I fitted a new bathroom (upstairs) radiator and since then there is a horn-like noise (as in, here come the Vikings) lasting for about 5 - 10 seconds at intemittent times (mainly when the heating turns off at night. It seems loudest upstairs, as if from the pipes above the boiler leading to radiator. The new bathroom radiator needs bleeding quite often but this does not get rid of the problem. Should I completely drain and re-fill the system? Any suggestions as to what it might be?

Unless its the washer oin the wc(!) its a washer in one of the pipes at the bottom of the baxi. Where the boiler's HW output gets to the union on the mounting frame. Turning the HW on would stop it.