Quattro 48/15000 Can anyone identify this sound? Is it normal?

Hi everyone,
I am an authorised Victron installer on Ibiza.
I’ve got a short clip of a sound coming from a Quattro 48/15000.
Installed around 4 years ago.
I’m not sure if it’s normal or if it indicates a problem.
Does anyone recognize this sound or know what might be causing it?
Any input would be appreciated!

[Attached the audio clip]

Thanks in advance!