Peugeot 308 noisy after warm up

Only way you'll find out is to get under it and check it out . Could even be a loose heat shield on the exhaust causing noise . Try blocking exhaust tailpipe by holding a cloth over the end should show up any leaks on system . Could also be a manifold gasket

Run it into any exhaust centre if your passing and they seem quiet and they will stick it on a ramp and check it out for you FOC