I believe someone is coming out to look at this but, given MC ways of speculating and knowledge too.. I'm curious to see if anyone has any theories as to what it is.
The closest I can describe the noise is like an air compressor running. It turns off when the power is shut down but then may start again randomly. It focuses on one radiator. in the study.
Actually, in between starting this and now, the plumber has visited and come up with a plausible suggestion. And it may be fixed.