So I had just started a new solo game on tranzit to add to the 70-80k points I had in the bank, nothing unusual, but during round 3 after I hit the box and got emp grenades, I was stuck with an empty pistol and was forced to knife and throw grenades. Now at this point my strategy was to get a good group of them going and blow them all up with a grenade, which I did, but 4-5 crawlers were left. So I went to work knifing each one and I got to about the 3rd zombie, I killed it, and a loud screeching/echoey noise rang throughout the map, does anyone have any idea if this is some Easter egg, or if it changes the game at all as I'm too terrified to continue playing if I just stumbled across a big secret, I want to play it out correctly. Thanks in advance guys :D