Should I replace propellers of my Mini 2 SE after some time eventhough there are no visible cracks or deep scratches ?
Atmospheric effects and the air friction itself is creating some color and texture changes on the propellers.
It's like a very smooth sanding on propellers. When I hear not pleasant, turbulent sounds from the drone while it's in the air I always think that maybe I should replace propellers.

Hello, there. Thank you for reaching out. It's recommended to check whether the propellers are broken, worn, misshapen, or corroded before each flight. If yes, replace them immediately. You may also consider replacing your old propellers for prevention and reliability purposes.

Please note: Do not use the old propellers again. Note to replace the pair of propellers instead of one single propeller. I hope this information helps. Thank you for your support.

That not pleasant, turbulent sound may not be the props but motor(s) noise instead.  Spin each motor manually and listen for any abnormal noises.  There isn't a stated timeframe to replace props so it is up to the pilot to do a visual and physical check before each flight.  Props are cheap so if you are worried just go ahead and put on all new props.

DowntownRDB Posted at 1-17 04:41
That not pleasant, turbulent sound may not be the props but motor(s) noise instead.  Spin each motor manually and listen for any abnormal noises.  There isn't a stated timeframe to replace props so it is up to the pilot to do a visual and physical check before each flight.  Props are cheap so if you are worried just go ahead and put on all new props.

Motors are not making weird noises. Propeller speeds are changing unnaturally in hover and creating a weird sound pattern and that's making me nervous.

Kopanz Posted at 1-17 05:02
Motors are not making weird noises. Propeller speeds are changing unnaturally in hover and creating a weird sound pattern and that's making me nervous.

As posted, if you have any doubt, just replace them. Cheap insurance. There are no guidelines or a specific number of flights or hours regarding prop replacement but they can develop nicks and/or hairline cracks that could be hard to see. You mentioned some " discoloring" ...yeah, just change them all.