If they engaged the AP on the miss it's the AP trim motor that runs the stab. It runs slowly and it is normal to hear it during that circumstance. It would be trimming nose down almost certainly. If they engaged it during a highly dynamic situation the system can't keep up often times and begins pitch oscillations. If they turned off the AP in response to a pitch up oscillation with the AP still trimming nose down, they might have got a big pitch over surprise as the AP disengaged. We all need more information obviously. Just throwing this out there.