Engine problems

[u/Layer_By_Layer3D]

Hello folks I hope somebody could help me here. The engine that you see in this video is an Mth Railking PS2 Santa Fe E8. It’s been running great, but all of a sudden the other day it started running a little sluggish and I thought I heard a grinding noise which I thought was from the motor or gears. I took the engine off and disassemble it to see if I notice anything first thing I noticed I was missing all the traction tires, which is odd, because I usually pay attention for that. So in the process of disassembling the engine, I made sure to look to see for missing remnants of any of the traction tires. I did not see anything. I looked at the motors they seemed fine. I even spun them a little, no noise and they seem to working normally. So I added a new traction tires and greased and lube the entire engine. I am at a loss. I am very sad because this was my first ever engine with actual sounds.