rear suspension

Anti roll bar bushes, they can look fine but be worn.

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.