I know you aren't talking about electric 4" mini grinders, but I have a bunch of them I got for 5-10 bucks each at yard sales and the only one that has croaked is the one Chicom one (likely HF) the others grind on... -A couple have needed grese in the gears but that's it... (B&D, Makita, Bosch, DEwalt, Metabo, Ryobi, Milwaukee.. 12 all told - I'm a believer in one wheel, one grinder, and switch grinders not "wheels" and now I have enough for a few at the main vise and the rest at the welding/fab bench)
See, thats the way Hitchcock's "The Birds" should have ended!
I'm surprised you decimated them so well; Crows are smart and will move on... In my last rented home, I used to pop them of a big tall tree top (I got tired of their noise), using a .22 rifle and CB caps (almost silent) and this was in the middle of the city. I body shot them and they would fly away when hit (CB's are very accurate, and I had this one zeroed for range, so no misses that might land somewhere bad.) Two houses on the same property and I could not bee seen from my "firing position" by neighbors. I don't know if I was killing them as their feathers are decent armor to something as light as a CB cap, but if they died they were far away. It didn't take them long to move a half dozen houses away or more...
Just treat that groan as "audio diagnostics" and if it changes, start looking for the issue...