You can't get to it from below. There is a full floor under the tub.
You say you hear it. I assume you are using compressed air because you could not hear a leak in the waste water system.
If you can locate both ends of the pipe in question I would cut it out and feed a replacement pipe. Nothing says you have to use the same path. It may be easier to reroute the new pipe.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles