The Master had heard the drumming since his childhood look into the hole in time. But I thought the persistent, insistent quality of the drumming that we saw the Master enduring first as Professor Yana, & later when played by John Simm was not part of the original symptoms. Instead, I took it that the signal became amplified by the Time Lords attempt to contact him from the time lock by tuning into his unique brain waves & reflecting them back at him. This would explain why we never saw the Master seemingly troubled by it in the Pertwee to McCoy eras. It was much less of a chronic, acute symptom, then, & only became so from the time the Time Lords attempted to contact the Master, which, in the latter's time line, was from his days as Professor Yana, though to the last time we saw him.
