Very loud groan/droning sound from back end - help?

[desert_porsche]

Hey guys, wondering if anyone has any ideas or suggestions for a problem I've had for over a year: my all-stock 2001 Boxster S Manual with 45K miles produces a VERY LOUD low pitched sound at medium to high speeds. The best words I can think of to describe the sound is a constant pitch drone or groaning. It does not sound like clunking, clicking, vibrating, pulsing, or screaching. The loudness of the drone correlates to the speed of the vehicle, not the RPMs of the engine (i.e. at highway speed, I can take it out of gear and let the engine go to idle RPMs, but this has zero impact on the loudness of the noise). Based on this, I don't believe the noise is related to the engine, exhaust, or anything driven by the serpentine belt. Everything else I can think of:

• Noise started about 1.5 years ago
• 1 year ago, I found that both rear axle CV joint boots had torn. I removed and rebuilt both axles. This had zero effect on the noise.
• 1 year ago, I replaced the differential fluid. The fluid looked normal, zero particles or metal found in the used fluid. This had zero effect on the noise.
• A couple of weeks ago, I noticed the tread on my rear tires was low, so I had brand new tires installed. No effect on noise.
• I don't think it's a wheel bearing because the noise definitely does not sound like metal on metal.
• It sounds like it's coming from the rear of the car.
• I can't hear the noise at all below 40MPH. Starting at 40MPH, the noise becomes perceptible and the volume increases with increasing speed.
• There is no noticable vibration at all in the cabin

What does that even leave as a possibility? The differential going out? Something else wrong with a rear axle? A suspension component lose or broken? I'm really at my wits end here. Has anyone else experienced this before?