Noisy roar sound in the rear 89 aero

Hi,
I was driving the other day, and at higher speeds (over 55) I could hear a really loud rumble sound near the rear... muffer looks ok on the outside,
nothing is loose... it is a walker 100 dollar mufffler that is 1 year old. (their best they made) it seemed to be ok at 60 or 65... but at 70mph... it is there... could this be a suspension (bad springs in the rear) issue (car is tilted slightly) 70 is the fastest I ever drive it, but I'd still like to know what it is... Thanks
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