Loud noises coming from underneath your vehicle can typically indicate a mechanical problem with the car, but sometimes the issue isn't all that involved. It's tempting to think that a malfunctioning muffler may be at fault, but it may instead just be a loose heat shield.
To check this, turn off your engine and put your car up on jackstands. Wait at least an hour after you have turned off the engine before getting underneath the car. Shimmy beneath the passenger side, and look for the exhaust pipe. The pipe should run along underneath the passenger's seat. This is where you'll find the heat shield, which is between the pipe and the car's floor.
Essentially, you'll want to attempt to recreate the sound you were hearing. Push, pull and shake the heat shield to see if it moves or rattles. If so, you'll simply need to tighten it back into place. These shields can come loose during driving, or potentially even bent if you drive over something like a large rock. You might just need to bend it back into place with some pliers.
If not, then at least you've eliminated one potential cause of the noise.