Good one.

When owned my M3 I had something like this. My situation was that each time I stopped I'd getting a large popping sound and I'd feel a thud under the drivers seat. I was completely convinced that something was loose under the car, because the combination of the noise and the thud made it really seem that way. Turns out there was a golf ball underneath my seat that would roll forward each time I stopped and bang into one of the metal seat rails. I felt like such an idiot when I discovered it, but like you I was relieved that it wasn't anything serious.