I have a 2005 LS, V8. I recently had a CEL that signaled a failed catalytic converter. Local Ford dealer attempted to replace the entire catalytic converter assembly, both left and right and associated tubes. They tried two different Magnaflow assemblies. Problems with angle of converter body and/or flange mating. The third was Walker, I think. They gave up when the third assembly wouldn't fit and sent it out to a muffler shop that could bend pipe. The exhaust moans and buzzes at various rpm's... even a slight howl. Any advice on how to get back to a smooth growl?