Purpose-built Boominator PCB project

Bit of a thread resurrection here! I have restarted my boominator mini project, so I set up a prototype to make sure the amp works as I expected. So it all works well although I've not tested the solar power in nor the LEDs.

One thing I am interested in is using a R-Pi and Hifiberry combination to provide the audio source (wireless in the garden). This uses the 5v power out.

I tried one old Pi/hifiberry combo and it worked well until I realised it's not wireless (doh)!

So I tried a new Pi/hifiberry combo that has USB-C power input. I've wired a "standard" USB extension lead to the +5v and gnd by chopping off the male end.

The Pi boots and works and plays music, but is noisy. High pitched and a bit scrunchy. It's a background noise which is covered up when the music is playing. If I power the Pi from its mains charger there is no noise.

The older Pi that has a micro-usb connection was also noise free.

If there is anyone with any thoughts, I'm all ears, as it were! Thanks in advance.
Graham