What's this?

[Folkranger]

I had another thread running "what's this noise". for the past month or so I have had this noise and couldn't find the source. It even ran while the 12 volt and mains were off. When it eventually stopped, (coincidentally maybe) one of the Goldschmitt legs would not actuate. The leg was near where the noise was so possibly connected.
This is a photo of the top of the leg. Is this a solenoid and could it have been oscillating?

 

[rb62]

Safety valve ?

 

[Folkranger]

And moved on today. Would you believe the leg is now working. Still no noise.

It gets worse, I've just come across a post of mine from July last year. Same noise, same fault-finding routine, same outcome (stumped).
The thing is I can't remember going through the same motions two years running.