Looks like some type of ringing. But impossible to tell what caused it now.
Sadly too many analogue to digital conversions are being done with little care.
I’d go the mask route but it’s a shame as it will impact quality.
Will wait until the archivist is back from holidays to ask him to recheck the original tapes, hopefully won’t have to mask and now I know It can be referred to as ‘ringing’.
Happy to have a conversation. Directly with them. If that helps.
Find me on linked in or via my https://www.vet.co.uk website.
Dealing with analogue formats was an area of expertise for me.