so i finally replaced the front left motor on my 5inch after that tree incident last week. used a spare 2306 2450kv i had sitting around, same spec as the others, and now i'm getting this low frequency vibration that wasnt there before. not terrible but you can see it in the footage and blackbox is picking it up pretty clearly.

ive rebalanced the prop on that arm like three times now, swapped to a different prop, even tried nudging the motor screws tighter in case it was a mounting thing. still there. the motor itself spins fine when i do the motor test in betaflight, no grinding or anything, all four sound pretty similar to my ear. rpm filters are already on.

is this just one of those things where you have to do a full pids retune after swapping a motor? or could the mount itself be slightly bent from the crash that i'm not seeing? frame looked fine visually but i didnt check it super carefully tbh

check the motor mount itself with a straight edge or even just your fingernail across the screw holes ? you'd be surprised how often the arm takes a hairline crack or gets slightly torqued and you just cant see it without really looking. also did you check if the bell on the replacement motor has any wobble? even new motors can come with slightly out of true bells especially the cheaper ones. just spin it by hand and look at the top of the bell while its rotating, you'll see it pretty quickly if somethings off.

and yeah after a crash i usually do at least a light retune anyway, the pids from before might just not be quite right anymore if anything shifted even slightly.

had almost the exact same thing happen and it turned out one of the motor screws was just barely too long and was touching the stator. like one or two threads too long. swapped to the correct length m3 screws and vibration was gone. might be worth checking what screws you used for the replacement motor vs what the original had.