In case it helps anyone chasing buzzes etc I have found them to be quite common on the C courses on mandoloncellos I have used.
The action and spacing on those needs to be very high by comparison with anything else I've seen. You need a real slope on the bridge to lift them high enough to be able to drive into the pair with any force and not have them slap each other and the fretboard. The distance they travel is very large by comparison with other string courses and they have a pretty large mass which takes a while to get settled in sympathy with it's partner. They start off out of sync due to the pick pulling them quite far before releasing them sequentially.
This was a real bother to me initially, and until I make a new bridge with a wider spacing and steeper slope across the G & C courses, the best compromise I have found is to use the 0.0740" C from the D'Addario string set, i would actuall prefer to go for something even bigger to get more tension in there. The problem with the J78s is every A course pair I've tried has not been able to come up to tension witout snapping on a 26" scale so I have to buy 18s for that. Certainly the spacing being even across courses makes little sense to me on a mandoloncello.
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