If it is AC then the wallwart will just be a transformer. As Lehmann are german made I wonder if the transformer is more suited to 220V mains?
I think I have a 20V AC wallwart that is rated at 240V input.
Yes def 21v ac - and yes Firebottle I think you could be right ref 220v despite labelled as 240. I will check it in a different location as the socket is mounted in a boarded wall which might be adding to the noise!