Good exhaust shops can 'tune' the exhaust when they make them. Normally they more the drone to a rev range that isn't used for long periods. A balance pipe placed in the right spot may fix it, or you may need something more drastic such as moving or replacing the mufflers. The hard part is finding a good exhaust shop that will fix it, rather than a generic one (midas etc) that will just want to sell you a new exhaust.
