On Sun, 14 Apr 2019 08:00:12 +0100, Brian Gaff wrote:

its affected by proximity, sounds more like a parasitic oscillation of
some sort.

That's what I'd be looking for. Monitor the cuurent draw, does it
follow the buzz level? The buzz is a symptom of the PSU running out
of steam, do the driver chips/transistors get warm/hot?

As it takes a while to appear and does so slowly I'd guess that it's
only just unstable and requires a signal to kick it off.

--
Cheers
Dave.