Hey guys. I just did a 3 inch X-pipe system on my Challenger with Spintech Sportsman Street 3000 mufflers. The car is Thunderous at idle, love it, and sounds incredible at WOT. The problem is at cruising speeds. I get a horrible drone from 1700-2500 rpm especially going up a slight grade. As I get higher in rpm, say 2700+ and 45mph+ it seems to quieten right down. I think the drone is comming from the front pipes in the system (between the headers and the X. Bad thumping type of drone and all I can think of is that it may be a scavenging effect causing the exhaust to echo inside the pipes. Any ideas on this?
3 inch system all the way back from my 500ish horsepower 451.
What are you guys running(or used to run) for X pipes?

Thanks.
Don.