dynomax vs. dynomax ...

I installed Ultraflows on my car this past year to replace some Flowmaster 50s. I made the change primarily because the Ultraflows have far less backpressure than the Flowmasters and are actually a bit less raspy / harsh sounding. I like the more mellow tone and the decreased back pressure is significant. The only issue I have run into is that I now have an irratating harmonic drone at 3200-3400 rpm. I also installed an H pipe at the same time so I'm not sure if the H pipe or the mufflers is causing the drone but it is quite anoying.