It's amazing how much dirt gets on a computer over time. It's not like we play with these things like we did our Tonka trucks! (My poor Tonka backhoe was subjected to mud, sand, the works...still have it in my attic but definitely not in mint collector's condition...)
Anyhow, that's one great looking machine. As far as noise--to answer your theories:
1. The SE/30 never came with the squirrel cage fan. Apple stopped using them sometime in early 1988 (based on manufacture dates; it may have been late 1987) so unless someone swapped out your analog board/fan for an older one, it wouldn't have one. All SE/30s were made between 1989-1991, well after the noisy fans were gone.
2. The hard drive is a definite possibility, especially if it's the stock Quantum drive (original ProDrive, pre-LPS/ELS). I've replaced plenty of these with an LPS (usually pulled from some type of LC) for performance, reliability, and noise reasons. If it's an 80MB original ProDrive, good luck--these things fail with the best of them! (Actually, I've had bad luck with all Quantum 80MBs regardless of type--maybe just coincidental?)
A good test would be to pull the hard drive power cable and see if the noise improves. If not, you're looking at a fan problem (and probably not the presence of the squirrel cage). However, this seems very unlikely and I'd say there's a 95% chance you have an old, noisy hard drive. Check the make and model of the drive while you're in there. (Silverlining can do this too if you aren't keen on opening your Mac and have a copy of the program). Out of curiosity, what is the manufacture date on your SE/30?