Lenasia vw

Brought a master car from vw lenasia and since then ive had a issue with a knocking noise which ive taken up with the dealer,car taken in twice before for the same noise but its a different story everytime as to what it is or whats causing it,now my problem is if the dealer did a 80 point check how is it that sound wasn't picked up then before they sold it to me,then when i rotated my tyres back to front i found out ive got 5 wheel weights on 1 rim as the rim is slightly buckled , please anybody help or advise me on what to do