Even though the noise was not air entering the CRT, I don't think you can completely rule out a cracked CRT, though. If the CRT had air in it, I imagine the analog would make some weird noises.

This little Mac must have been really jarred to knock off the video board and bend pins in the process. This much force could have easily cracked it... Most, if not all CRTs make a quick little hiss or crackle noise when you first turn them on. Are you sure this isn't what you're hearing?