I attached the clock to a lab power supply and gave it 12v and then adjusted the voltage down. Interestingly at about 11v it is considerably quieter and then gets louder again towards 9v. I also notice that it is quieter when not under load.
I really don't remember it being this noisy and putting my ear close I can perceive what might be described as fizzing or at a stretch crackling.
Perhaps I should preemptively replace the input electrolytic? (Which is a PITA since it is SMD flavour)
Just checked my schematic and I'm pretty sure I ended up with...
"EEEFTH101XAP - SMD Aluminium Electrolytic Capacitor, Radial Can - SMD, 100 µF, 50 V, 0.34 ohm, FT Series" on the input side. That looks like a good choice... not sure I have any spares.