strange fluttering sound
 

just the other day I noticed a strange fluttering sound and I'm not sure what it is

it sounds like it's just in the exhaust, not a leak it is coming out of the tail pipes but only sometimes
I was thinking maybe something came loose inside one of my dynomax mufflers but they only have about 15000 miles on them and it sounds like it is coming out of both sides

I can't hear it when I rev the engine because the exhaust gets too loud, I mostly notice it when I'm rolling to a stop

also I don't notice any loss in performance either

any ideas guys?