exhaust problems???

[clionut172]

just a quick one on my ph1 172.
after new engine went in 4k later found that when in revise.
seems to be a knocking sound comming from the exhaust. i think it is comming from the pipe near the cat the part with to springs on.. do you think i neeed to replace it or could it be something else? when the car is cold it blows round that area. but soon as it is warm it goes away. its the only thing wrong with the car and doing my head in lol..
cheers