[Eliot Ness]

I suspect, as you do, that there was a rubber piece between the two U brackets. If it isn't too hard to remove one of the shafts it would be easy enough to cut a rubber piece to fit. Use something like a thick chassis washer or whatever you can find in your shop or local hardware store.

If it is hard to remove a shaft, then experiment with different sizes of rubber cut to shape and try wedging a piece in between the brackets; that might require a softer piece of rubber to be pliable enough.

You could try putting a piece of tape part-way around the two U brackets and fill the space with RTV Silicone, but I'm not sure how well it would hold up, I suspect it might not do very well.