Pesky road noise - can't locate source.

[Manolito]

1997 328i 5sp 295k miles

Speed-dependent road noise, mildly pulsating roar as though I were driving a truck, just keeps getting louder as the weeks go by.

Completely independent of load. Accelerating, decelerating, coasting has no affect on volume or pitch of noise. Volume and pitch increases with speed. At 20 mph it is noticeable. At 80 mph it is quite annoying. Seems completely independent of road surface.

Rear wheel bearings were replaced about 50k miles ago, and they feel fine.

Front wheel bearings feel perfectly tight.

Replaced all four tires with new Michelin Defenders. That eliminated a low speed whomp-whomp--whomp that turned out to be a cord separation in the front left tire, but otherwise had negligible impact on the road noise.

Changed fluid in transmission and rear end. No change at all in the noise.

I thought that the only thing that would be speed-dependent but load independent would be the driveshaft carrier bearing. I bought a new one, but when I put the car up and exposed the driveshaft, I found that the rear U-joint was bad. Didn't figure the U-joint was my noise, as that would be sensitive to load, but needed to fix it. Ordered rebuilt driveshaft which came with a new carrier bearing. Put that in today. Absolutely no change in the noise.

I have been searching the web, and rear shock mounts are often mentioned as possible suspects for road noise, but people rarely post what their problem turned out to be, and I would think that noise from failed shock mounts would be very road surface dependent.

What am I missing here?