@FearUS
The rumbling stopped probably because Eren no longer has access to the founding titan's power (Zeke). You make a good point that Eren no longer need Zeke in the first place because he "convinced" Ymir to help him, but I guess the found titan has a rule that needed a royal eldian blood anyway. EDIT : living royal eldian blood, dead Zeke's blood seems useless.
When Eren first used coordinate, it was when he punched Dina's titan, so no rule broken.
Dunno if the rumbling titan is similar to any normal titan. Titans were in 2 categories in the beginning, normal titan and abnormal titan, then a 3rd category comes in with titan user. Maybe the rumbling titan is 4th type that requires found titan to move?