^not sure about your pump, but I am running my Koolance PMP-450S daily at 24V (which is its max) and it's quiet enough for me (fans are louder). I'd set yours (12V model) to 5 and see if it's OK and leave it there.
D5 Strong has no vario and by default at 12V it runs comparably to around setting 4-4.5 on vario models, mine was dead silent at that.
D5 Strong has no vario and by default at 12V it runs comparably to around setting 4-4.5 on vario models, mine was dead silent at that.


















