Not a nice sound whatever it is.
If it's a water pump problem, are these the ones where the impeller can break off? Could be that?
Had a noise recently in the c5 shitroad, the same but much worse, convinced myself it was a cam chain tensioner pad that had fallen off, took the top off one side of the engine (not a ten minute job on a 2.7t) and found out, there was no power steering fluid in the fekr
So you never know.
But going by the frequency sounds too frequent to be a single tappet or little end? Maybe not though, they (pistons) are doing a fair number of strokes at even low revs.