After replacing a radiator at the weekend I now have a strange vibration noise somewhere. This noise occurs when the boiler is on

Our central heating system seem a little strange to me. We have a boiler (gas) in the kitchen which controls the hot water & heating. However we also have an emersion heater (electric) in a cupboard upstairs (this is never switched on but I believe the boiler still pushes the hot water through this as it gets hot when the boilers fired up). In the attic there are two water tanks, a large one and a small one. I temporarily disabled the smaller one and drained the upstairs radiators in order to replace the bathroom radiator. I then re-enabled the smaller tank and released the air from the upstairs radiators as they were re-filling.

The noise seems to be coming from either the emersion heater or the boiler. Although they're on different floors there's only 3 or 4 feet between them. I also believe there's a water pump hidden away behind the emersion.

Any suggestions or tip on what to check would be appreciated.

PS - Could there be air trapped in the boiler, pump or emersion?