I just finished diagnosing an X555 with the same issue. The problem was with the connection between the motherboard and the HDD/Audio daughterboard. When pressure is applied to the top of the case, it would flex and cause some pins to stop making contact (I noticed the power/status lights would not work when pressure was applied either).
This is not a great design choice by ASUS and I hope to fix the machine by replacing the screws that were missing from the rear of the case which should make it more rigid and reduce the amount of flex.