I never owned a Rustler, but I have had a sqeuling noise before.
Check to see if your slipper is adjusted properly and check that the wheels turn without making any crunching noises in the gear box. It could be a bad bearing also.
Ultimately, I would not be lazy and take the gear box apart to inspect everything. Check all of the gears and bearings. Also check out the motor and clean it. Check the wheel bearings too. Bearings may need to be cleaned and re-lubed or it could be too late for that and you may need new ones.
Doing maintenance is part of the hobby. Your Rustler will last longer, go faster, and handle better if it is properly maintained. The instruction manual that came with your Rustler should be a huge help to you-check it out!