Thanks Lance, did not know that .
In firmwares before 3.4.6. I never had that problem .
And I would never suspect it since my Altitude hold hovering I think it is quite exactly in the middle of the gas stick but it might be not so exact.
In fact immediately after switching in Autotune mode I receive a audio message on my Taranis with FRskyMAvling (Cloony version) , "Autotune started" , something like "waiting for .... ...." but I'm half deaf and my english knowledge is poor so I do not understand the message.
In Mission Planner "Message" Tab , only "Autotune start" appears.
If the audio message received is related to autotune it would be nice to have it appearing also in Mission Planner.