Developed Knocking Noise after replacing the pads HELP

Hey guys,

I've recently replaced the rear brake pads on my car, and it developed some kind of knocking noise when I brake just moderately hard.

I took the car back to the technician who did the work and he checked to make sure it's not caused by a nail in the tires. He didn't notice any issues, so he just took the pads off and put them back on. The problem went away for a couple days but now it's back again.

I don't feel too confident about taking the car back to the shop that did the job; they couldn't figure out what the issue was the first time, so I don't expect them to fix it.

If anyone had similar issues, or know what could possibly caused the problem, please let me know!

Thanks!