Occasional hum

There are two types of mains frequency noise: 1. 50Hz hum; 2. 100Hz buzz. In addition there is interference noise from TV and monitor screens which may have LCD (or similar) excitation frequencies, sometimes 75Hz.

There can be contact noise where a connection is on its last legs, thus removing the shielding function intermittently, resulting in "mains hum".

As you only recently noticed it ("as I noticed it yesterday when listening to vinyl"), then is there anything that has changed recently, or any new household electronics or electricals?

It is not new, but yesterday on headphone listening it was there.

I had heard it before using my speakers, but was surprised to hear it yesterday.

I think I would go for option 2. 100Hz buzz.

Nothing has changed recently, but I know I have noisy mains, as we have a smart meter, power line network adapters, air conditioning and a heat pump.

So if there is nothing I can do to filter it out, I guess I will have to live with it.

Not a case of living with it. More like an ongoing investigation.

I and others can only give examples. Here's another.

I had an Hadcock GH 228 unipivot arm and upgraded to a GH 242.

Rather than being quiet, the new arm would introduce mains hum/buzz every now and then.

This was because the arm tube metal wasn't making electrical contact with the arm base, because the unipivot wasn't pressing "hard enough" together to always cause an electrical contact. Otherwise, the unipivot was doing its job as a bearing.

As such, the arm tube surrounding the arm wires wasn't an effective sheild, and then it was, and then it wasn't.

Eventually I said sod it and bought a fuss-free Technics.

We can only give you our stories in the hope something clicks (or doesn't...).