Take the mufflers off and drive the car. See if the sounds still there.
Sorry I just found this thread, you should have emailed me. My wagon is catless, stock exhaust from the DP back, with 2 washers between the Y pipe flange and the mufflers.
Some have said the exhaust tips may be to short also, they may need to be further past the bumper.
My Spec has stock exhuast less mufflers, no drone. The tips on the muffler deletes extend a bit further then stock.
174,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. Also CHECK YOUR OIL, these cars use it.