the other side of the garage.
The true story was that one of the garage door springs broke. There are two springs per door and they help the motor lift it. The sound that I heard was the sound of the broken spring swinging down and hitting the inside of the garage door. Metal on metal.
We found out quickly that without springs, garage doors are REALLY heavy. We were running late for a party when this happened and the car was trapped. Luke had to hold the door up (and hold his breath) while I drove out.
We found out from the friendly folks at Home Depot that you should never replace just one spring (will cause a ratcheting effect when the door lifts) and that you must know how much your door weighs in order to buy new springs (if you get the wrong weight spring for the door, you'll burn out your motor). But finding out how much the garage door weighs is surprisingly easy: just put your bathroom scale under it. Wa-hoo. Ours weighed 100 lbs, and we needed to be within 15 pounds. The closest spring weight was 110 lbs. so that's what we got.

(The old springs. The one on the left is the broken one. You can see it doesn't have its loopy end anymore)
We made some mistakes while we tried to replace the springs, and now the right things to do seem really obvious. For instance, these are springs we're dealing with. When the door is down, the spring is stretched and taut and dangerous (if they break, or come unhooked). When the door is up the spring is relaxed, it's easy to work with (wait, wait, Physical Science 110 is coming back to me...something about kinetic, no potential? energy...maybe not).
We started removing the other spring, like they told us to, when the door was down. Luke's job was loosening the bolt that attached; my job was to hold on the spring to slow it down and keep it from flying across the garage. I'm standing there holding this stretched out spring, thinking "What is going to happen? This thing is gonna kill me when it goes!" Then I realized, "Hold on, I don't have to die, we could just lift the door up (thus shrinking the spring back to its relaxed state)." Whew!
So we opened the door, but we had to find something to prop it open with while we worked. Like a box spring. And the case for an electric sander. Hey, we're not picky.
New springs nowadays come with a safety cable that runs through the center of the spring; so if it breaks again, the spring won't come crashing down. The cable will catch it instead. It was hard to figure out where I was supposed to attach the cable, but I think we achieved at least a reasonable facsimile of correctness. And the door opens now!
Funny story: At one point we had our old car inside the garage on the broken garage door's side. Normally it lives on the other side, but we switched them because we needed to be able to get the good car in and out easily. But the old car died, inside the garage, while the door was broken. It was a Catch-22. We couldn't fix the garage door unless we got the car out of the way, and we couldn't get the car out of the way unless we fixed the garage door. Eventually we used the propping method while I steered and Luke pushed the car out onto the driveway. It was a pretty sketchy plan (considering the likelihood of the car knocking down the prop on its way out and crashing down on my husband) but it got the job done.

(Two new springs installed. You can see the safety cables running out of them in to the left)