Mr Green,
I experience sleep paralysis more then I care to count. After a while, you start to become akin to the signals that come before the actual paralysis -
this is what is important.
By letting yourself keep going with the paralysis, I think you will be quite surprised in the outcome. This is pretty much my HOWTO with astral
projection - by using sleep paralysis as a catalyst, I am able to "disconnect" when the opportunity arises.
As for the sounds you hear, that loud rumbling - almost train like sound to me - always comes before such events. You may also experience what seems
like people talking in your ear, being able to see through your eyelids, or a feeling of a sudden drop. Learn to deal with these feelings.
Do not get scared - if you do, you will ruin it - and just ride it out until you "pop" out the other side - literally. Enjoy.

