Detonation sounds like a can of spray paint with metal balls inside. It's more prevalent at WOT.
Many people describe the sound in different ways.
I bet what you're hearing is the natural "cackling" of the exhaust when you let off the gas. Which is at the opposite time that detonation happens (when you are ON the gas).
If you don't have a supercharged/turbocharged engine, the chances that you're getting detonation are VERY VERY slim. The computer is designed to prevent that. GNX was right, the computer would be pulling timing so quickly that you'd never even be able to hear the detonation.
I'll bet'cha dollars to donuts that you're hearing something else. If you want to double-check, you can always go on down to your local tuner/dyno-shop, and have him put an O2 sensor on your exhaust (or inside your tailpipe), then get a good tune. It's worth the money anyway. You'll pick up a couple of extra horsepower, and your worries about detonation can be relieved.
Until you can get the tune, if you're worried that it is detonation (despite my EXPERT opinion that it's not haha), use premium fuel and don't go WOT if you can help it.
Whenever you figure out what it is, please post back here, I'm interested to know what you find out.