Video sounds like you are tapping on a rock (or possibly but less likely a piece of steel) that is imbedded in the vertical passage somewhere above the horizontal exit. This is a fairly common problem in cleaning out those old passageways. I cleaned out a number of them years ago. I fabricated a long drill bit from a steel rod, flattened the tip and sharpened the edges. Imbedded rocks are the toughest to get out but lots of tapping, drilling and patience always worked in the end. I wouldn’t drill the drain/weep hole higher up. Very risky and it will advertise the problem you are having.
