Hello and welcome to JustAnswer!
The first thing I would do is make sure all hangers are in place and intact and not allowing the pipes to reverberate excessively, rubber hangers will act as a vibration damper.
Also make sure that the exhaust system isn't contacting any part of the vehicle other than the hangers.
If everything is in place then this is going to just be a characteristic of this system. What you could do is have another small muffler or resonator installed in the system to drop it's volume, or you could remove this muffler and install a different one. I've seen some issues like this on some vehicles with Magnaflow systems, a different brand of muffler will change the sound.