as above - rust spots, possibly from rust thats eaten through the metal pipe? if thats the case then the pipe needs replacing.
what is the pipe - rainwater or soil and vent, and where does it come from and go to?
your soil stack will not be discharging into the soil -
soil stacks drain to manholes and then the main sewage line - or go to a tank.
dont connect the rainwater discharge to any soil stack unless the alternatives are not feasible.
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