What motors have you previously used in the car ?
Could be normal cogging associated with a sensorless motor
What battery are you using ?
What start power do you have configured in the ESC ?
Do you have a 3s battery to try ?
Easy enough to test if its bec related
Plug in another power source to the RX (Receiver pack etc...) and disable the internal bec
If it goes away problem is a loaded bec
Cheers