Buzzing when door is opened

Hi, I’ve just started getting a buzzing when a door is opened in the van (T6). It can last from 30secs - 1min+ and seems to becoming from near the speaker just to the right of the steering wheel but I don’t think it is the speaker. The van has been converted to a camper a few years ago but no electrical work has been done recently. Any ideas out there? Thanks!
 
Are they power open/close doors? I think that area is where the door motor actually is.
 
There's a thread here on that motor that might give some context:

 
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