Rattling noise from transmission?
 

In the middle of a transmission + diff fluid change. Not sure what was in it before, but putting Motul 300 75w90 (non LS) in both now.

I have the car raised up and all four plugs installed after refilling with fresh oil. I turned the car on to let the gears spin for a bit in lieu of driving up and down the street.

And if the wheel speed is around 10+ mph (as in: I couldn't hear/feel this rattling at idle in first or second gears), I hear a weird rattle and can feel intermittent resistance in the drivetrain.

In the following video there are two distinct groups of noise that both sound like the engine. Start by listening for the low frequency rumble of the exhaust from the start of the video until about 20 seconds in. Note that the exhaust sounds very consistent, not like the RPM is jumping all over the place from a rough tune.
Then you can clearly hear the abnormal rattle coming from the transmission area (restart the video and watch to 20 seconds again), much louder than the exhaust note when the mic is on the underside of the car.

I've never run the car while lifted like this. Maybe this has been happening for ages, and the momentum of the car has prevented me from noticing it while driving?

I'm just headed outside to top up both oil levels. Looking forward to hearing what you guys think it could be.

Thanks,
David

Video: https://photos.app.goo.gl/VeNfyPdEZshKrYc69

Mods:
Engine is stock. Car is built for SCCA Solo Spec Coupe, meaning coilovers, sway bars, wheels/tires, catback, brake pads.

Normal normal normal Factory fluid is thick to mute the lash of all the disengaged gears.