Found a DWV cast iron pipe with small rust bubbles. Looks like pin holes that rusted and sealed themselves back up. So it’s not actively leaking. Any plumbers out there have more info on this? I assume it could be much worse on the inside.
As you know cast iron rusts from the inside out, typically.
That needs replacement as it will do nothing but get worse until it fails completely.
Recommend a qualified plumber make corrections and assess the balance of the cast iron DWV with repairs/replacements made, as needed, to correct the structural integrity of the damaged cast iron, or some such statement.
Don’t tap on it. Corrosion and moving water are powerful, persistent forces and will wear through cast iron piping over time. Looks like it’s had its day.
Recommend replacement. Cast-iron pipe only has a 50 year lifespan.