I recently completed my hexacopter (Pixhawk, AC 3.2.1). Test flights in Stabilize, AltHold and PosHold were acceptable to me ( a beginner), other than the fact that every once in a while the copter would jump up 1 or 2 meters. These flight were all done in my yard, so I maintained a height of about 3 meters and flew slowly. I check the logs and vibrations etc. seemed OK (to me - anyway).
Today I took the copter to a large field to attempt an AutoTune. Before I started the Tune, I flew around with a little more "vigor" than I had in the past. I was almost immediately rewarded with a vertical jump of about 3-4 meters. I was able to get her back down with no problem. Over the next 6 minutes this happened 3 more times. Looking at the log, these jumps took place a 1+ minutes, 2 min, 3-1/2 min, and 7 min.
The log seems to show that the problem was caused by AccZ vibrations. If I am correct, my question is, what could be causing the vibrations and what steps should I take to eliminate them? I have checked everything. The Pixhawk sits on Kyosho Zeal (sp). It all looks OK and I have followed all the usual instructions.