I'm going to assume its an automatic transmission. Is the millage higher than 50,000? If so try changing the transmission fluid.
One clunk when accelerating (but now repeated clunks) and another when braking can be caused by broken motor mounts.
You have either run something over or your car is due for a service
Possibly a wheel bearing
You can use a car noise database to compare noises at mycarmakesnoise.com
Vroom Vroom
Check under your car... you may have lost a muffler.
it does this because it just does
chiken
engine
Get bin Laiden
Depends on where the noise is whats making the noise. Can you be more descriptive of where and what?
tie rod