so I have encoders on my two motors and when i drive the robot the motors work fine, although they make a different sound. But why do my motors lock when im not driving them?
The back plate/encoder housing might be too tight
I’ve noticed that the newer motors from this year tend to sound different and lock up internally more than older motors did. Not sure why, but my hypothesis, based on pure speculation, is tighter tolerances on the new internal gears which would be a good thing.
Edit: Misread, didn’t notice that you had encoders on. I have heard that they can be finicky. Either way the above still holds true lol
The locking happens because the encoders provide friction. The black block on the back of the motor has hundreds of ticks per revolution, and every tick makes the motor a little harder to turn. So the motor must work several hundred times harder to pass the same amount of area. If you need less friction use a red encoder because it only has half the number of tics per revolution so the friction will be lower.
I seriously doubt this is true. The encodersjust record how many ticks the motor turns. They are designed to not make much friction. I have experienced the locking even without encoders, and I think that @917F is correct about the internal tolerances
They’re oxptical encoders, not physical ones. I’ve had a few lock up before because I had the back on too tightly
How exactly do you know this? Ticks per revolution should have no effect on friction, it is the extra gears being introduced which does. And still this does not force the motor to work hundreds of times harder.
@Swordman03 - If you see this locking up while the robot is connect to the joystick, this may be because of the way that your joysticks are programed. Most teams that I have seen program their joystick to incrementally increase the speed of the robot as the joysticks are pushed further. However, thanks to small manufacturing defects, not all joysticks are centered at the point 0,0. This leads to a small amount of power sent to the motor. How these teams have solved this problem is by increasing the dead-zone on the joystick, or an are where the joystick will have no effect on the motor.
As others have already said, this is not true at all. Before replying to a thread, please make sure you know the subject matter.
As for the topic of the thread, it could be lots of friction on the axle that is small enough for the motor to move, but large enough to make us moving it by hand difficult.
I’m pretty sure that it is the pink encoder that has half of the number of tics. I’ve used it in the past and it works great!
If nothing that’s already been suggested works I would try checking your motor gears(the internal gears). If one of them has gotten some chipped teeth it could possible be causing a lock up of the motor(maybe? I’ve never had a lock up from chipped teeth but I guess anything’s possible). A chipped tooth could also be causing a weird sound like what you said(It’s sort of like a grinding sound if I remember right). So while that is extremely unlikely it could be worth a look 
I’ve heard secondhand that the axle holding the black and white gear that you replace when you install an encoder is sometimes too short. I assume that the axle gets dislodged and then the gears bind or cause that noise. My coach said that you can replace that tiny axle with one from an older motor.
Are you applying any form of P, PI, PID, or some sort of control loop onto your motors using the encoder? It could possibly be that one of the motors is pushing on a surface for too long where the motor starts to tighten up.