Actually, even though the Helmholtz resonators work on reducing drone, the drone isn't really caused by the exhaust.
Drone is caused by the frequency produced at those lower rpms and the natural resonant frequency of the interior air space of the car are tuned to one another, reinforcing those frequencies, making them louder. And if you notice, the drone comes back at double those rpms, around 4000 to 5000. The drone at low rpms is like the first harmonics, the drone at higher rpms are like the second harmonics. This is because the internal air volume of the car interior allows those particular frequencies to propagate - a.k.a. - drone.
Ever notice the drone decreases drastically to almost non-existent when you have the windows rolled down? That's because you just changed the "tuned" resonant frequency of the interior by opening the windows.
Has a trusted friend or family member drive off with your car as you stand outside and listen. I bet you won't hear any drone. I bet the exhaust will sound just about a loud at those low rpms (2000 - 2800) as it does at higher rpms (3000 - 5000).
A very similar thing happens in a room with bass notes on a stereo system. It's called "room gain" and "room modes" which have both peaks and nulls. Different room sizes will have different room modes where different bass frequencies are allowed to propagate near the wall boundaries making them louder or possibly quieter. In certain areas along the walls and within the interior of the room, you'll notice the bass frequencies are louder than normal, typically near the corners (a.k.a. - corner loading).
Anyway, now I'm starting to ramble, so I'll stop there. But my point is this, it's NOT the exhaust system that's droning, it's the enclosed interior of the car that's causing the drone.
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