I was in the bath and suddenly the boiler starting making a low whinning noise which started to get louder. My partner want all round the house trying to find out where the noise was and it appears to be coming from the boiler or maybe the pipes its hard to tell.
When the water is turned on (hot or cold) the high pitched noise starts again.
The boiler is behind our fire (a back boiler). Can anyone help? (We are in Kent, UK)
Probably an air pocket somewhere in the plumbing. Probably before where the branch off to the hot water system *(\heat exchanger, tank, etc.) sinc you said you hear the whistle when you turn on cold water.
Not sure how to get rid of the air pocket for sure although running the cold water full blast for a few minutes is worth trying.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.