Hi Kalyan,
As described in this post, increasing of the current control bandwidth can lead to instability of static operation, while an increase in the voltage ramp can. Before switching to an octocopter together with the autopilot, you should first test at least one axis on the test bench in the entire range of speeds and setpoint steps. I want to emphasize that the maximum voltage ramp and bandwidth values are best checked on the test bench. These values may vary depending on the size of the propeller, motor, etc.
The annoying sounds at some speed values in static operation may be due to the big current control bandwidth value.
The RPM control mode has two main advantages. First, it can improve responsiveness by increasing the order of the system. Secondly, in this mode, the system is less susceptible to the influence of load disturbances caused, for example, by gusts of wind.
But there are also disadvantages. In particular, the controller is more difficult to be tuned in this mode.
This mode has no effect on energy efficiency.