could be fan?
problem is peolple cant be specific when it comes to diagnosing strange noises/sights/smells etc..
one mans drone might be another mans whining might be another mans humming might be another mans....
etc etc
this might not mean much to the average punter but slight differences in sound could indicate different faults
thats where the skill in diagnosing comes from
being aware & taking in as much information as possible
to diagnose exact fault there is no substitute for standing in front of boiler
can only give you a best guess
AWOL
July 2012