Hi there,
I am new to the Fairphone family, bought the FF4 a few months ago and have been having the same issue from the start : the phone vibrates every second when listening to audio (and when WhatsApp is backuping).
I found a solution on a WhatsApp forum for Samsung :
Long press on the WhatsApp icon. Press ibutton / Notifications, and deactivate Media Playback
Hope it might help some other users 