Bowling ball noise in trunk
Have you removed the spare and all the other stuff under the cargo floor to see if that is the issue?
More to the point, have you removed the bowling ball from the trunk?
This could be a serious suspension problem. Get the car to a shop ASAP.
Hitting the lane?
Making a strike?
Your really do need to have a mechanic check this out. It could easily be serious enough to suddenly get a lot worse and cause you to lose control.
It might help to provide more info as to year, miles, and so on. It could be benign such as a loose spare tire or something far more serious such as a bushing in a knuckle or trailing arm. Whatever the cause, it should not be difficult to sort out.
Remove everything in the trunk including the spare, the jack and luggage platform. Go for a ride and see how it goes. If the noise is still there, you have a problem.Look for loose exhaust ,heat shields and broken suspension parts. Report back