You could try a piece of stainless flex tube in the straight section before the muffler. Gm had us do that on some v6s with a droan at 1700 rpms. If the droan goes away when someone pushes down on the exhaust pipe the flex pipe will fix it.

 

update.....today I welded on some turn downs that are not only directed down but towards the side as well......worked perfect, sounds good in my opinion. Im a happy camper.