sometimes i have heard things i heard earlier the same day
no idea why, but i have even heard my cellphone ring while it's not..

too much coffee maybe

Just seconds before you fall asleep sounds dimmish and just as you start to come alert again even the very faintest sound around you seem to be magnified x10

I used to work and go to college full time and their was many times I almost fell asleep at the wheel on my 70mile commute back home from college.

I can remember catching myself falling asleep to many times to the extream loud rush of wind blowing over the car or the harsh noise of my wheels running off the road, which in a normal circumstance is not loud at all.

I have heard explanations.
Some of which describe what you are hearing as your still conscious mind picking up on the shutting down of your otherwise conscious functions.

I posted about something similar long ago.
During the dropping off point, if you are inclined to remain partiality conscious, all types of weirdness can occur.

Just to add to this....

If you ever hear voices as you are about to fall asleep or have just fallen asleep, and it sounds so loud as if they are in your head- that can be your guardian or a spirit speaking with you.

It happens to me all the time, and it always freaks me out. Its never fun to hear something that shouldnt be there. I agree with the poster above that it sounds like it was a ripping noise of your spirit leaving your body.

What you need to be concerned about, is when the voice makes you feel like something is wrong or if there is a scream involved. Once I had a female spirit scream inside my head and I thought I was going to # myself. I sat straight up and screamed myself, my husband thought I was totally freaking out. If this happens, you should try to clear your house of any spirits. You dont need that kind of drama when your trying to sleep!!!

when you do hear the noise, just concentrate on hearing it and staying with it....it will increase in sound, and what happens next it depends. I see the spinning tunnel, and again, I try to stay with it because if i panick, it goes away. My sister doesn't see the tunnel, she just 'pops out' and gets visions. I don't get visions, but I get something like a dream state where I'm flying, but I'm aware at the same time of whats going on, and I get to see different places or people.