is this doing it only when the car is running and a distance away from you? There could be some inteferance from any number of things. Check the wiring, and make sure you have fully charged, or fresh batteries. Hope this helps.
Actually, the pipe on the RTR is composite. But it still sounds like metal to metal interference. That's a tough one on the NRS4. A significant portion of the car is plastic. I'll have to bring mine in for a minute and look it over....
May want to also check the antenna wire, the battery wire and all the servo wires to see if any got stripped. Just a thought.
Have you ever slammed your car into anythingat a high speed? -Curb? -Pole? -Tire? Check your chassis for distortion. It's possible you've got a tweaked chassis (bent or slightly distorted). If you do, well, good luck because it's almost better to buy a new one. You can try to lay it on a flat surface and take a mallet and beat it out, but if you're not carefull you'll just cause more problems.
Also, check your camber and toe-in on all 4 sides. If they are off enough, it could cause your car to pull hard to one side under accelleration. Check your manual and reset them all to factory specs.
Check it out and let us know. Good luck.