I have this issue with my PH3S i bought 2nd hand on ebay, came with a 3rd party battery and a geniuen.

This issue has happened a few times while outside and insides, ive spent about an hour inside trying to replicate the issue, with no sucess but i have a recording of it, it sounds like the motor is struggling as it trys to self correct, but this doesn't Alwasy happen. *happens on startup only and doesnt fix till i restart the drone*

There are no errors shown on the dji go app either

Hi there, thank you for reaching out and we're sorry to hear about the trouble. With regards to that may I ask have you tried to calibrate the IMU, Compass and the Gimbal of the Aircraft? You may give a try to do that. However, if the issue still persists. Unfortunately, what I can only recommend is send it into the facility for further checking of the issue. You may just go to this link https://www.dji.com/support
Best regards!

I think you may calibrate the IMU first on an even level place and then the gimbal. If this not help then the gimbals sensors stucked and provide wrong values to the gimbal without send a message to the mobile about that.
It does it in midflight too. Very mysterious.

I think you have a bad ribbon cable on the gimbal.  Had a similar problem, replaced the ribbon cable and took care of the issue.

That occasionally  happens.  The grub screw must tighten to the flat side of shaft.