Exhaust drone/resonance on highway

Been driving the ZHP more lately as our daily drivers get service. I've noticed ever since I bought the car back in 2019 that cruising between 76-80 mph (very specific range) there is a resonant drone from the rear of the car. I assumed this was exhaust drone due to the ZHP muffler. It goes away if I let my foot off the accelerator. If I downshift to 5th at the same speed, it is there, but didn't seem as loud. It was slightly louder prior to flashing the DME with the updated software to fix the SES light issue.

I now commute over an hour to work, so I had plenty of time to think about this today. This seems odd to be normal, especially for BMW. Do other people notice this with their cars? And if not, maybe what I think is an exhaust drone is nose from the final drive? If that is the case, it may need a fluid change.

Strange, I just did over 1300 miles with much of it in the same speed range you mention and I had no drone. Stranger still that it's not RPM dependent, but rather speed dependent.

How many miles on your diff fluid? Wonder if it could be a rubber diff mount that's on its way out.

Changed diff fluid 2 years and 5k miles ago. Noise was there before and after. Its a little rpm dependent, as it is less loud in 5th than 6th. Coasting, there's no noise. No one else hears a reverberating hum 76-80 mph?

Would love to try to isolate this. It's pretty maddening to me. I'm wondering if it's tire noise that only hits at that speed? My dad has an E36 M3 that has a strange whine from the final drive in first gear (always has). Silent after that. Tolerable though because you don't spend much time in first gear :)