WHAT AM I?

I am something, an alarm, maybe, but I don't pop up on your screen.

source

I am not large, but I am sane, only if someone else wants me to be.

I can reassure you, or scare you.

I make no sound, but I am loud, sometimes hidden, sometimes proud.

I am made of a word associated with the sound that time makes.

I remind you of the open sea, and crash of the waves.

I am hard to ignore, even more difficult to leave.

But once you do,

Then you are free

What am I?