My honda civic 04 automatic makes a rattle like sound when I turn the key off?

I noticed that when i turn the key to the off position some where under my streering wheel I can here a little rattle like sound. It last about 1- 2 seconds
(i hear the usual normal cooling down sound the engine makes after I turn it off. So I don't think its the engine cooling down )
Any ideas what it could be?

It may be the engine "dieseling" (running with ignition from heat and compression rather than spark) for a couple seconds before the fuel cutoff kills it. Not a huge deal but may be cured by timing adjustment or other tuneup-type tinkering. Might also respond to a higher grade of gas.