spoilers 9.12

Doctor: Clara, there’s a sound you’ve been living with every day of your life, but you’ve learned not to hear.

Clara:  What sound? What’s wrong? Just tell me. Doctor, what sound?

Doctor:Your heartbeat. Your physical processes have been time looped… Frozen between one heartbeat and the next. Even your breathing is just a habit. You don’t need it.

The first time Clara met the Doctor, she had to hold her breath in order to not being discovered. She listened her heart beating.
That loud pulse is gone now, the same pulse which a very long time ago could be fatal for her.
It’s curious the fact that the very first time the Doctor met Clara she was breathing, with a fast pulse. Then he left her as a human almost dead, with no heartbeat and breath.