Loud rhythmic humming on stacked Quatro 10kw inverters

[Westphal_Solar (Perry Westphal)]

Hi, I have a humming sound on a QUATTRO 48/15000.
Is it the same?

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!