My car is making a clunking sound

I drive a 2007 Mazda 3i touring edition. Early last week, I heard a clunking sound in the back right corner of the car. The sound is only apparent when driving on bumpy roads, or going over speed bumps. It sounds like I'm carrying a loose bucket in the back of the car.

Incidentally, my brakes make a scraping sound when slowing down. The mechanic said that this is normal for the type of brakes I have on the car. I'm not sure if this is related to my clunk problem, but I wanted to disclose anyway.

Also, out of habit, I put my handbrakes up. Sometimes I drive off without putting it back down. I'm not sure if this also relates to my current problem.

It'd be great if you can help isolate the problems for me and give a rough estimate of what the repair would cost.

Thanks much!