All,
I have read this thread and researched this issue across the web. I recently purchased a 2nd hand P3P that had this issue, and I already had a good working P3P that has the latest firmware. I tried all of the recommendations here except the props, I know it's not the props because I swapped them between the two P3P's. My theory from reading all of this is that DJI swapped suppliers on a board, controller, or chip at some point and the new firmware affects some but not others. Let me be clear, the affected drone flies flawlessly. This problem is ONLY at idle when the drone is on a flat surface.
My observation: At startup, the P3P powers up normally and after motor initialization about 10 seconds will pass when I hear the motor pitch raise up a bit, and the vibration begins in all 4 arms. I've tried different surfaces underneath the drone and then I discovered the most amazing thing. I read a post here that someone suggested holding the landers and the vibration would stop- he attributed this to Gain. So, I took two small squares (6" x 6") of thin foam, about 1" thick and put it under each side of the landing pads, restarted the drone, and guess what...the vibration completely goes away. I then cut two pieces of the foam to the width of the landers, zip tied it on, and tested the unit on every surface I could find. NO Vibration. Remove foam, vibration is back. Amazing...a nickels worth of foam makes a $1200 piece of electronics settle down. How's that for engineering.
If I was spitballing causes I'd say that there is some change in sensitivity within the firmware, and a board or controller is over-compensating for the feedback coming through the drone. I would hope, as many of you also do, that DJI is reading this, taking notes, and will reflect this in the next firmware release.
Try the foam or another soft cushioned surface under yours, I'm curious if this can be repeated.
My apologies for the long post, but I am sure this has more than a few people going crazy.