Yeah, it's hitting something, somewhere.

Barge cement? Qu'est-ce que c'est? That fix (with any of the materials mentioned) strikes me as assuming one has the skill in applying them to get a layer down that is almost absolutely even--and spilling out the sides would be a big problem. Lot of fussing around.

I'd start by replacing the string. It should be apparent if it's happening, perhaps the wrapping has come loose somewhere on the old string and is vibrating back and forth somewhere under the bars?