To double check - you are checking the ATF fill with the engine running, after shifting through all gears? It will be under-filled if not.

Belts? If you can get someone to rev it for you, or you manipulate the cables, perhaps you can reproduce it in the garage/driveway and neck it down more. May or may not be AT as indicated.

Not the lexus, but my SUV has done something just like this but at lower revs and under load since 2010. I am a pretty decent mechanic. In that time I have rebuilt the ps pump, rebuilt alternator, done t-belt and attendant belts and puleys, greased and checked all Us and slip joint, serviced all hoses, induction, PCV and EGR valves, transfer case fluid, probably A LOTelse.. I even rebuilt the transmission, and was sure that would finally solve it. And it still does it. No clue at this point.

If you get no where on it, I would at a minimum throw a bottle of Lubegard red in the AT. It might fix it, and if not, it will buy you valuable protection on that somewhat fragile U140 transmission (I can’t recall if it was 99-01 that had it, or 98-01).

Do the Lubegard, it’s not “mouse milk” - eg, snake oil. Before you do the next AT drain/fill, try some SeaFoam or B-12 Chemtool for a week before hand. I hate adding either to a sealed system as both are quite strong (B12 more than SF), but they can clean up gunk on solenoids and valve bodies that can make issues. If you are REALLY sure it’s the AT, try a few ounces of SeaFoam in the AT, since you just did fluid and won’t do anything to it soon.

I had a bad noise with my PS system when I first got my SUV (Acura SLX, Isuzu OEM) in 2010. It had 135k neglected miles on it then. I added a few ounces of SeaFoam to the PS, ran it a week. Noise gone in two days. Flushed the system and it was fine until last year. I rebuilt the PS pump because the bearing was noisy, different than original noise, but the SeaFoam cleaned up the sticking/constricting issues in the pump for seven years until something else made me re-do the system.

So, this “add this, add that” advice may sound like lazy advice, but it really does work because there are clearance issues with sub-components that neglected fluids do not address.