I assume the iodé build for FP6 is based on ArianK16a’s repo for LineageOS support. It’s a known problem already. See current state issue and in discussion in fairphone forum.
Workarounds are configure the alert slider to none or disabling vibration in settings.
For me it was not easy to recognize first. The random vibration is so low, that you nearly don’t feel it if you hold the device in hand. I noticed it only when laying it on a solid table, then you hear it clearly.