Offside rear knocking

Hi all, I have a ongoing problem with my insignia 2010 cdi diesel estate. I have a knocking from my offside rear wheel,when I start moving it sound like a consistent knocking and speeds up as I exceIarate,but as soon as I put the brakes on slowly too slow down it goes silent. After I take foot off it starts again. I have taken it too a garage and he tried too find the problem but said he can't put his finger on the problem,,, mmmm. He tried replacing rear offside shock, front discs, why he replaced them I don't no as I told him it was a rear problem,the constant knocking stopped but when I go over uneven Road or bumps or holes it knocks, but now it is constant again, has anybody had the same problem or any help on the problem. Love my car but its wrecking my head. Cheers, hopefully I can get it sorted. Thank you all