I had a problem like that. I never did figure out why. If you change esc to another kind you will see a difference. The esc was sent back to bk. I haven't got it back yet.

I just got back from the track and the motor been making a grinding sound on and off when it goes into very low rpm but i didn't notice any loss of power. Does anybody know what's going on or what i can do about it. Thanks.