ORIGINAL: Max_Power....... it makes me believe it is definitely through the muffler seals ........... Todd
Then change the seals?
If it is the modern collector ring plus side canister muffler and not the older and smaller plain collector ring then check if the cooling jacket fins have been relieved around the the contact area of the canister.
And obviously check that nothing is squashed or damaged.
Good luck, and photos may be good to push this one further.