2006 350z. 20"wheels athermarket exhaust, bagged and several more upgraded toys. When we ride there is a loud buzz or roar that is going on. It does it at all speeds. Louder as you go faster. Lessens when you slow down. Has direct corresponding tone with speed. I even got the car going and switched to neutral and cut motor off. Still does it. What can it be.

 

Without actually hearing the sound or ability to put hands on, difficult to diagnose. Is it from the front or rear? Is the car in neutral and/or clutch depressed?

Would suggest checking the following speed dependent items (since you already shut the car off test):

Tires, wheel bearings, brakes, transmission (if it's still making noise with clutch depressed/neutral).

 

My money is on rear wheel bearings