exhaust leak...knock

[55gasser]

i have a 05 6.0 that has a loud churp (knock) only when its warmed up.

i had a mechanic from ford that lives down the road look at it & he thinks its an exhaust leak ?

it comes from the back of the block on top.

wouldnt a exhaust leak do it all the time ? intsead of when its warme up ?

no noise at all when u first start it, takes about 10 min.

thanks for everyones time & help