Clunking noise - 2010 Ford Mustang

noise underneath sounds like a clunck only on fough roads not on smooth roads

You have worn suspension part(s). Lower ball joint, struts, outer tie rod end could cause this amount other things. Needs to be brought in for repair and alignment.

A knock, or a clunk? I’d suspect stabilizer bar links as a first guess. They tend to wear on these cars. Moog has a nice replacement that is grease-able. I’d check the front stabilizer bar’s bushings while you are under there. Lower ball joints are the next spot to check. Upper strut mounts next. Thye are actually pretty robust so I wouldn’t expect issues but it is possible.