I had that somewhat even with an H-pipe. Switched to an X-pipe. That fixed it. I like the tone now, too. Can hardly hear the exhaust from within the car at cruise.
I have a Summit system on my Duster (Factory manifolds with 2.5" duals with Summit mufflers & talpipes) and since I changed the rear axle ratio (2.45 -> 3.55) 2 days ago the car drones so bad on the highway after a one hour drive I can't stand it.
Will try a H or X pipe first to see if that changes the frequency point.
One step forward with the gears, 3 back with new exhaust drone, new driveshaft vibration and new alternator howl at cruise rpm.
I had that somewhat even with an H-pipe. Switched to an X-pipe. That fixed it. 
I like the tone now, too. Can hardly hear the exhaust from within the car at cruise. 
