My NC250 and NPX300 were humming along quite irritatingly on Sunday evening. Quite audible as I was listening quietly. The sound also seemed off.
Fridge, washing machine and tumble drier all on. Turned them all off sequentially. Slightly better…. But I’m used to a silent system.
This am I realised that the heated towel rail in the bathroom upstairs was on as it is put on when the central heating is off. The key is the warmer weather has resulted in a ‘half power’ towel rail. Hence the humming.
Went and turned it off altogether. No more humming.
Posting as a tip to others - look for heaters not fully on and quite some way from the hi fi!
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Does the towel rail still cause hum when it’s on full power? It could be switching in the controller that is the problem.
Appliances with variable power settings are known to be problematic for causing humming. A while back my 552PS starting humming out of nowhere, then I noticed my daughter was using the hairdryer. When she was done I asked her to run it for ten seconds, turn it off for ten, run it for another ten. Sure enough, the 552PS dutifully hummed along when the hairdryer was running and stayed silent when it wasn’t.
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Hairdryers and fridge freezers are the worst in my house.
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It’s probably time to look at installing dedicated mains, with a separate consumer unit. Then you can have cosy towels and better sound too. It’s one of the best value upgrades out there.
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Is your system on a ring main? 
Electric heating elements are notorious for causing high quality toroidal transformers to hum. Glad you found the cause.
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The hi fi is on a ring main downstairs with nothing else on it. We don’t have a TV, sky box or any such. The kitchen and upstairs are on separate rings. So yes I was a bit surprised.
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Is there a way to suppress the effect at source? My system is quiet most of the time, but hums occasionally. I suspect one of our fridge freezers is the cause. Is there something I can fit to its power cord to suppress this?
Ideally you want a dedicated mains circuit in a separate consumer unit from the rest of the mains wiring. Otherwise all the circuits are linked to each other on the bus bar.
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@BrendanD - Well done - on you persistence and pursuit of the cause.
@Corry - Hairdryers are a known, often cited cause.
Even then if there is impedance in the supply to the Henly blocks before the two CUs, the hum will still occur.
The only answer may be a ‘DC blocker’… it all depends on where the impedances are.
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Easier just to turn the offending heater off…
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My SN3 hummed pretty loud when my wife ironed clothes upstairs.
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So, no big deal … now you’ll be drying yourselves with damp towels in the summer. Just like camping in humid weather. 
I once traced my Pass Labs mono blocks transformer hum to a thermostat in a tiny heater in an upstairs bedroom a long way from my hifi. I think the affect is called magnostriction, dc current in the AC.
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Even that does not stop my wife’s hairdryer from giving my gear the ‘hum’. Generally my gear has more and longer periods of increased hum than in the past. I think it’s the main grid - not my house appliances (apart from the haidryer).
I have a dedicated mains from the consumer unit but even that doesn’t prevent hair dryer hum in my Supercap2. Interestingly none of the 250DRs or either of the two Supercap DRs seem affected though.
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It certainly can be. There could be other people on your same run from the substation adding to the effect. E.g. may be a small manufacturing business, or someone with a swimming pool, etc.
The warm summer months can be troublesome.
Living in a city/urban area you only have to walk around outside to hear all manner of ambient mains born noise.
Air conditioning, service systems overloading.
I live just next to residential retirement apartments which has its own outboard power station. Often heard as a low drone rumble and pulse. (I’m sure that raises my noise floor)
I once found my amp jumped to overload oscillation triggered by the dishwasher making a fuss because it was empty of salt.
Housekeeping has many levels. 