1993 BMW 325i...When I kill the engine there is an almost inaudible humming sound that comes from the center console for a few minutes after the car is turned off.

I can't differentiate if it originates from the vent or from the Stereo. Sometimes, very rarely, it makes a gurgling sound which leads me to believe its coming from the vent.

This happens with the fan on or off, the stereo on or off, whether I've drived a long or short distance. I can't find any cause-effect relationship. The occasional gurgling doesn't seem to be triggered by trip duration, engine heat or anything else I can think of.

The only other observation I've made that would seem to apply is that when I eject a CD, it is usually very hot, hotter than I've observed in other cars I've driven.

Well to begin with, I think the noise is coming from the evaporator or another a/c component. The excessive heat from the radio deck is probably a combination of the in car amplifier (if you have one) and the heater core being located directly behind it.
Max

So should I just disregard it?

Yes, you should, but the noise you're hearing is probably the cooling fan for the heater control unit. It runs for a few minutes after the car turned off. It drove me nuts too until I figured out what it was.

I've got the same thing and thought it may have something to do with the air con? I've ignored it for the 12 years I've had the car!

I have a 2000 323i and it makes the same noise. So this is normal right?

You have demons lurking in your vehicle.