How do you know if your Catalytic converter's are going bad?

Question

My cats are making a weird rattling sound but there isn't any weird odors or anything so I'm really confused!

Is it possible that they are just loose?

Also how much do catalytic converter's cost, I would install them myself.

Answer 1

they make '''fits all'' cats for around 100

Answer 2

the ceramic internals of the cat are shafted...it wont pass the next mot....dearler cats can cost £500 plus,you may as someone else mentioned gat a ''universal'' cat...me,personally i would replace it with an original one from a car brakers off the same model...