Your rear pinion gear has lost a tooth. You need the Heavy Duty Final Gear Set, HPI part 86336. About $30 at our local hobby store.

We have the MT2 18SS and have gone through 3 (THREE) Ring and pinions in three months. [:@][:@] This is UNBELIEVABLE!! [:'(] I cannot believe the terrible quality of the ring and pinion in this car. And we're not even racing the thing! I know how to set up a rear end and followed the instructions perfectly. Needless to say, I am NOT happy. I like the engineering of HPI, but they've made a huge mistake in the Ring and Pinion of this car. I will be writing them to talk about it.

Brian - Boise ID.