Not knowing the plumbing layout, I can only suggest a couple of ways to go about this. First I would try plugging the pipe when it rains, if this solves the problem then your trap is going dry, this could be caused by vent blockage, trap going dry by a wicking action ( as lint or something in the trap soaking the water and drawing it out to the waste line. You could hire a plumber to smoke test the system, this could possibly locate a broken pipe etc.