Right now??

I hear Edguy blasting out of my Studio Monitors which is still too quiet to drown out the sound of my Rig being its usual Uber Powerful self...

Natures nice but I do love the sound of a Powerful Rig ticking over!

whirr whirr whirr whirr as the 2 harddrives beat with each other.

My laptop is off, so I don't hear an angry "clack" every $RANDOM*3 hours as the actuators do scary stuff.

Preheating the exchanger? I found the hard way this winter, by putting the air conditioner on a timer to turn on an hour before I come to bed back at the lodge after night shift, that there is such a thing as too cold. Perhaps the heat exchanger froze up or something. But -1 degree or so, very cold wind blowing directly into the heat exchanger, and I heard it trying to start repeatedly, but it never got anywhere. Silly me thought that with the air conditioner having sufficient time to heat up the place before I got back, I wouldn't need to also put the electric blanket on.

Heh, I used to hear a beat pattern with the old case. A regular ...thruuuummmmm......thruuuummmmm......thruuuummmmm... from a couple of fans in there.

Re heat exchanger: combi-boilers have an internal heat exchanger for on-demand hot water (the central heating water is diverted through this). Some like to run for a few seconds every couple of hours to keep this warm so that you get hot water 'faster' when turning the tap on. It's slightly disconcerting when you first have one of these, and you hear it fire up for what seems like no reason.

I hear fan-whine, since Chimaera only very rarely actually powers off. Tone changes very slightly every so often, as the fan controller picks up on temperature variation.

Of course, the clicky-clicky of my battered old warhorse keyboard, now I'm bothering to listen while replying.

My housemate's girlfriend, sighing through the over-thin wall.

The rain on the windows, and the periodic sounds of digestion as my computer flushes buffers to disk.

That's outside. Within, between the ears, I hear other things, but they matter less to the world.

Computer noises, Sophie talking to her mum in the other room, clattering from the kitchen downstairs.

All the sounds of my desktop - Fans, and my keyboard.
Traffic.
Central heating clunking a bit.
A door opening, then closing, keys clinking, and then footsteps going away.
I can just about hear the bass of some music, I think it's a from few rooms across on the floor above.
And in about 3 minutes I'll hear the bell - the clock chimes every 15 mins.

Ironically, I hear my laptop. I wish I could get it half as quiet as my desktop is.