[BuckyK]

Well my first piece of advice would be to never own a townhouse. I never understood the idea of buying property that you don't actually own.

You need to call a plumber. They will be able to help you out more than anyone can on this forum since they will be able to dig into the problem.

Since the smell only happens after it rains would mean that the storm line is backing up (either it's clogged, or the wrong line size was installed). If the wrong line size was installed, then you're responsible for the new line size if it runs under your property. If it's clogged, then you're responsible. However, your HOA dues may be able to cover the costs for you since technically all you own are from the studs in, and not anything outside the studs or below grade.

Check all your p-traps. These are meant to hold water at the bottom of the loop to make sure sewer gases don't come into your home.

No matter what, you're will need to call a plumber. Expect to pay at least $100 for him to diagnose the problem. Hand the the bill to the HOA if it's under slab or outside your house.