Strange noise comming from my monster

[A RC Dude]

I just ran it a few moments ago and about 5 mins into the run, i started to hear a winding noise but it was only wile staying the same speed or coasting. i heard it comming from around the motor and i took the pinion off and rolled the car down my driveway at a high speed a few times and never heard it but when i put the pinion back on and reset the mesh, it started again.

any idea what it is? i'm thinking it's a bad bearing but am not sure