I have a 280 LB robot, and I am using two SM34165DT motors with a 9:1 gear ratio to create a differential drive mobile robot.

The motors, currently on MDS mode, are relatively quite when running on a completely flat surface. But I noticed that when the robot has to make a lot of effort to move, for example up a hill, it makes a high pitch noise (which a little different from the grinding noise produced by a poorly tuned PID controller).

I even try moving the robot in torque mode: it has a bigger acceleration, but I pushed against the robot and it makes the same high pitch noise, but at a lower volume. And after a certain point of pushing it, the robot won't move because the force I'm exerting against it is too high. I'm assuming the lower volume of the noise is because the PID is not activated in torque mode.

This might be due to the inertial load, but I still need to prove that numerically.
I've been tuning the PID, but the noise is still the same.

In the mean time, is there an option that I can turn off in order to reduce that high pitch noise?

A high pitched noise is an indication that KD is too high (a low pitch usually means KI is too high). If you have to use a large value for KD to get good motion, then the inertia mismatch is probably too high, meaning you need a higher gear ratio to compensate.

Another indication that the inertia or load is the problem is seeing the over current bit being set.

I see, but I also get the same noise when the motors are in torque mode, except that the volume a little lower.

That makes me wonder if part of the noise is coming from the gearhead.