From the advice during previous attempt at figuring out the solution to the vibration issue, I built an ARF X500 quad with pixhawk 6C mini.
I’m however observing the same symtoms as on my custom frame: significant vibration increase during 30 degree forward flight compared to stationary hover.
Make sure your motors are level. Here’s a tool I made to help with the X500:
Every little bit helps with those. Also make sure the quad is balanced.
The vibration is tied to speed, so it’s aerodynamic. Have a close inspection of the props. Make sure they are balanced and that there is no tip damage. Make sure the tip paths are lined up. If those are the thread-on props there might not be much you can do with them (and the tool I linked above won’t work)
Also look around the airframe and make sure nothing is flopping around loose. At 17m/s/ or 60 km/h things might start buzzing or causing issues.
At the end of the day it’s an X500. They’re great platforms but they aren’t the sleakest drones out there. That’s a big carbon plate being pushed sideways through the air.