Just about everyplace that you see a mounting point where things go together (e.g. at the shock absorbers (lower & upper mounting), pivot bushing bolt, axle beam pivot) the are bushings - often rubber. They cushion things & take up vibrations. When they start to get old & hard & cracked they creak. Your control arm has the same kind of stuff.
There are no ball joints back there. In your original post you said “the mechanic said maybe…” It sounds like you asked about the creaking noise & s/he just threw out something off the top of his/her head. The way you wrote it didn’t sound like an actual diagnosis and without someone being able to actually inspect it you can’t really get one.