OK, if your drivetrain is making a sound kind of like that of a wounded bobcat at the YMCA, then that is a sure sign of something being wrong.
Some things to look for ( no matter how simple, they can still be overlooked
)
1. One of the motors isn't hooked up backwards, the leads don't need to be reversed if you are using the kitbot Boxes.
2. check the RPM of each motor to make sure that they are all spinning the same without a load
3. if you've seel any magic smoke, check the brushes on the motor... in some cases they can be fixed, or defenately replaced
4. If you are using the kitbot boxes, make sure that no set screws have come loose
5. Make sure you are calibrating your Speed controlers correctly ( we had trouble with this a little)
6.As was said, Y cables are a good Idea, even if they do somewhat clutter your electronics up
7. If you are using the Kitboxes with two motors, try running them with only one.
Just definately make sure that your motors are spinning in the right direction and at the same speed, because if you pit CIM against CIM, one usually over powers the other, producing the terrible noise you might have heard when you put it on the ground and damaging stuff.
Sorry if this didn't help, I gave it my best shot
-CodyC