Help diagnose 'clink' sound? video link

[Russianblue]

This sound happens when I put the car in reverse and also when accelerating normally, then let off gas (clink), the reapply the gas (clink). I can't tell if it is the front diff, transfer case or tranny.

https://youtu.be/CDn1Yk3BOsI

I am able to reproduce it while parked, as in the video, while shifting from park to reverse and back with my foot on the brake. Sounds exactly the same as it does when I'm driving. No slippage of any sort and everything seems to be working perfectly other than the noise.

It is possible my front diff is low on fluid. Could that be causing it? My other "best guess" was slack in the transfer case chain.

OR, it is just the transmission warming up for her death song? I hope not. I'd hoped to bequeath this to my kid this fall when she gets her license.

Thanks!

2003 e53 X5 with 177k. Original (lifetime) tranny fluid...never changed since new.