I would be looking at the turbo as the source of the noise.

 

Straight pipe will have more drone than a flow through muffler. Could try a different muffler? Ive become to enjoy the sound the Aero mufflers have. No drone with them either.

 

Try an FTP resonator or put the cat back on.

 

You need to add some back pressure not get rid of more.

 

might try adding some weight around the mid point. I eliminated my drone with 2 u muffler clamps and a 1/2" steel plate about 12" long and 5" wide. Just drill 4 holes to match the width of the u bolts and bolt it on with the clamps. First try and it was a success.
My exhaust system was a cut and weld modified 2007 Classic that used the original muffler but I left the CAT off. Wish I had just gutted the CAT and left it on.

 

I've got virtually no drone with my 5" straight pipe...

A buddy of mine just put an Aero Turbine 5050XL on his 5.9 cummins and it settled it down LOT. I'm looking into getting one for mine... At idle and high idle it kills almost all of the jet engine sound... We tested by stuffing it on the end of the tail pipe, lol

 

When I deleted I went with the 4" Flo-Pro exhaust. Horrible drone at 70mph.

I took a chance and replaced the muffler with a 31" FTE Resonator. The truck is quite literally as quiet as it was bone stock.

 

Thanks for the advice. Anyone know the size of the flopro 5" muffler? I don't want to have to cut the exhaust at all to add it I'd like a direct replacement. ]

 

There is a bit of info in their FAQ section.
FLO~PRO - Technical Support

"Our diesel mufflers are designed to eliminate "cab drone". For example, in a 5" system, the muffler has a 5" straight through perforated core packed in an 8" case with a high temperature fiber material. Mufflers have a case length of either 20" or 30".

 

a 1/4 wave resonator took. are of mine.

 

*took care of
not are.