i was reading this discusion and i thought my case is similar to yours
let me tell you my story perhaps it will answer your questions.
i recently bought the Xperia Z2 and i faced 2 main issues:
A- During calls, the other party cannot hear me well (microphone problem) ( even the .314 update did not solve it )
B- When answering calls the other party hears a vibration ( your case)
talking about problem B let me tell you when it happens and when it doesn t. if you answered while the phone is vibrating the vibration will continue and the caller will hear it, but if you answer between two vibrations the caller wont hear anything.
I assure you it is not your fault, it is sony's fault, i had couples of years ago Xperia P it had the SAME PROBLEM and when i bought the Z2 i assumed that SONY would have solved it. But i was surprised that the problem is always the same
in my opinion these small issues are reasons for loosing potential customers ( i am one of them) because i directly returned the phone ( i guess ill be getting the M8)
Hope i helped u