Crunching noise from transfer case happend 3 times to wifey today

[BMC530i]

Car "crunched" three times this morning while wife was driving, it drives ok now but i think i can hear a faint grinding sound coming from under the car while driving next to wall

It happened to me once last year while driving, crunching sounds from under the car, and vehicle slowed down on its own. I immediately shut the engine off and after restart i drove home without any issues. Whats options do i have now? The X5 3.0 has 74K and I'm suspecting the chain is getting stretched. I honestly feel that I'm driving on borrowed time here from now on.

Should i go ahead and order re manufactured NV 125 case (whats the best place to find one) or should I buy replacement chain and attempt a DIY?

Thanks in advance