here is a video of the popping/hissing sound of the engine....
http://www.youtube.com/watch?v=J_rDS5kDPds
had the same thing happen to a lly same sound everything yes it is a injector! not really pushed out but a broken o ring! just thought i would give my 2 cents on the sound.
i wanted to add just a little more went back at watched the video again,the o ring for the intake cant and will not make this sound,because it sounds to me,the only time it makes the sound is when the pistion is coming up on compression stroke.
to add more to why the intake wont make this sound,you should have boost "well a little bit" on the intake side,so if it was the o ring,in the y bridge,it would have the sound all the time. not a hit and miss sound.
now i have also had this happen,put the injectors in tightin them down and drive it,then it start making this soun as well,because it didnt seat good,just needed to tighten down a little more,this coudl be the same kinda deal.
again this is just my 2 cents thought it mite help the guys over at ppe,but i bet they dont need the help lol