If you are experiencing strong drone below 3k rpm, I'll bet you have an aftermarket mufflers that keep each exhaust system of 3 cylinders on each side of the flat-6 engine separate from one another, all the way out to the dual tips.
Does your rear muffler look like this?
If so, please consider some content I posted previously on another board:
"I went ahead and did some calculations [on a 986 exhaust system] using a tuned organ pipe harmonic equation, where I think the drone would be most affected by the presence (or lack) of a crossover that couples the two separate exhaust systems of the 3 cylinder banks. I measured (by eyeballing with measuring tape, and guesstimation) that there is in the ballpark of an 8 foot long overall exhaust path length, from the exhaust valve through the cylinder head ports, headers, and rest of exhaust system out to tailpipe. (That's 2.44 meters.) An assumption made is for the speed of sound of the exhaust gasses being 500 meters per second (based on an estimate that the average exhaust gas temperature would be about 500 F throughout the length of the system). And guess what frequency came up at being the 1st harmonic (the loudest and lowest)? 51hz at the exhaust valves per bank,
which equates to 2,040 crank rpm."
Now if you had the stock muffler, it basically has an internal crossover that basically allows the two banks to freely (without encountering any baffles or significant restrictions) allow the two separate banks to freely communicate sound pressure and rarefaction waves:
Due to the alternate-bank firing pattern of the Porsche flat-6 boxer engine design, this factory exhaust system results in 1st order, ~2,040Hz frequency being substantially cancelled out.