Sorry for digging up old thread, I was doing a search on Drone and came across this one.
I have some info to share from my previous experience at Holden, some of you may know about it, it's a really simple concept, cheap too.
Drone is a resonant frequency that unfortunately is aligned with a particular excitation frequency, such as engine firing order or power train or even roughness of road to a lesser extent. If the input frequency happens to be near or aligned with the resonant frequency of anything, that thing is going to be extremely excited and shake/vibrate a lot more than the input energy. If the resonant frequency is quickly surpassed, not many people will notice it or complain about it. But the issue is when it's close to a typical road speed / engine speed... like 60km/ 80km/ 100km/ etc...
An easy way to combat this is to move the resonant frequency higher or lower, there's 2 variables, Stiffness of the system or Mass of the system. it's much harder to change the stiffness of your Exhaust unless you're willing to add stiffening bars etc... so the easiest is the add mass. Adding mass will reduce the resonant freq. So keep that in mind, you won't get rid of your drone, you're shifting it. if you shift it low enough, e.g. below the idle speed of your engine, it will appear as though it's gone all together.
Exactly how much mass and where to add is hard to explain as we used equipment to find exactly the spot and exactly the mass... but it can be done by trial and error if you have the time. Jack the car up safely on jack stands, while car is in neutral bring the rpm to the usual drone rpm, get under the car, hold a metal pipe/screw driver/rod whatever that's stiff so your hands will feel the vibrations from the exhaust pipe, move along the exhaust pipe with your metal stick, and subjectively note the location of the MOST vibrations, Mark that location and then get some Metal Ring clips and test with add some weights, step it slowly by 250g or whatever you can find, until the drone has changed. sometimes there are multiple locations... the same process can be repeated.