7.2 whoooo's help please

[catman25]

ok running 6x 1.2 volts. radio shack nimh's and with the remote off it works fine.It works fine with the remote on too but it gives a rumbling sound.When i turn the wheel it goes away and then comes back.Just wondering whats going on and if its bad or just a fluctuation due to increased voltage .and help would be apreciated.btw motor doesnt get hot.either does the bottom of the car.thanks