eunos roadster knocking after new wheel and tyre change

hi, im new here, i have just put new 14" wheels with new tyres on my 1992 eunos roadster mx5 and now theres a knocking, i think its coming from the back does anyone know what this could be? are as anyone had this problem before? is it dangerous? i have no idea of machanics being a lady, so any help would be really grateful, thanks

sorted it one of my wheels was not set right 2 nuts had come loose, now ive lost my key for locking wheel nuts how dumb am i ....derrrrrrrrr

went to challenger tyres they got them off for me in 5 mins brilliant guys thanx