.5-.9 VAC & 0 VDC sounds like meter noise to me too, diabloNL.
I think that suggests pretty strongly that it's just a vibration, rather than any sort of electrical leakage. It probably shows up more markedly on the 230 VAC @ 50 Hz because of both the higher voltage, and the lower cycles -- that is, I also expect it's happening with 110 @ 6 Hz too, but as you say, it's just enough less pronounced that no one notices.
Whatever the deal actually is, it's good news that there's no electrical leakage.
