Put in a bigger stereo. Give it something to beat the drum to.
Seriously though, a hearse is often a customized version of a large luxury car (well, as far as I know). They have extra stress in places other cars don't and have stuff in places you never thought of because of their length and how they're setup to serve. It's normal for them to have thumps and bangs in unexpected places and times. It sounds like your hearse has some mileage on it (correct me if I'm mistaken) and older well used vehicles can do some odd stuff (I know), then include the 'unique-ness' of it being a hearse can make it a special ride.