I assume the sound is really load, then it is related to exhaust system, such as:
1. leaking in exhaust header. White smoke should be visible in engine compartment
2. cracking on exhaust pipe. White smoke should be visible along the pipe. Need to raise the car for visual inspection
3. crack gasket in resonator, or bad join. White
smoke visible. Raise car for visual inspection
4. rusted muffler. White smoke is visible around muffler
5. hit on muffler and open. White smoke is visible around muffler
Expect to spend $100 - $350 to repair. Normally occur to old cars > 10 yr old. US car may occur > 5 yr.
Highly recommend to repair due to:
1. Noise pollution
2. Gas pollution
3. Annoying to passengers in car
4. Engine oil may leak from the crack, or openning
5. Leaking gas will make surrounding area dirty, and hard/impossible to clean