if tyre is worn on edges trailing arm bush perhaps, or lower arm bushes maybe,,, if no tyre wear then like stu says drop links or d bushes on anti roll bar,, might also be a broken spring that would make a knock noise or shock mount maybe,, so much it could be best getting it looked at
Sometimes like with my 2008 R56 it was making a flat knock sound. What it turned out to be was the cone cylinder shaped plastic dust boot that goes on the upper part of the rear shock. What I had to do to correct it until I bought accordion dust boots; was cut out the part of the plastic dust boot that got damaged from being mis aligned during installation of my new shocks.
I would at least look at the upper plastic dust cover on the rear shocks.