I've noticed that before I get a chance to leave there is a pop or a wave in my head-So far I've experienced only waves,but last night I felt and heard a pop a bit behind my forehead,but I didnt manage to get out.So far all my attempts at willful exit are unsuccessful
You experienced, what most people here call a strobe. They can be sometimes very loud and strong and occur often together with a bright flash. These experiences are not a sign for an possible exit. When they occur, the relaxation is reduced. These experiences are often triggered by disturbing events (mostly noises, rarer by itches or touching).
What you have experienced may be Exploding head syndrome http://en.wikipedia.org/wiki/Exploding_head_syndrome Or at least something similar to it. I had exploding head syndrome two times a couple of years ago. It started when I started taking certain coenzymated vitamins. I heard and felt a very loud snap/explosion in the middle of my head justs before waking up. I was not practicing astral projection at the time. Considering you felt it in waves makes me think your more suited for astral projection than me and and those with EHS. Hope that helps.
Peace.
Yes I have heard and felt a loud popping before exiting. That is only 1 of some loud and crazy noises you will hear
That is common because it happens with me.