[Clarification after some of the answers given below: Specifically, the sound is a "thwap" "thwap" type of sound, repeated at irregular intervals. I have found 5 different mob spawners in my world, and have checked for the sound on peaceful and non-peaceful modes. The only places near my spawners and areas that I have completely cleared out that I hear the "spawner noise" is in a radius around the spawner. I hadn't done the experiment to determine range, figuring this was common knowledge. The initial answers below, lead me to believe it is not, or that I have a unique install.]
I have a long (1Km) minecart track, and I'm astonished at how many mob spawners I hear as I move along it. I'd like to find them, but have been unsuccessful in my searches. I suppose I could carve out a 17x17x17 region around my location. Seems excessive. Looking for something better.
On the wiki, I have found no indication as to the range at which a spanwer makes noise. Is it 17m Euclidean? (That is the activation range.)
Does the noise change as a function of range?
An obvious approach to finding them, once the range is known, would be to move around to several different places that are "on the edge" of the "noise envelope" and then find the center (using F3 and the coordinates therein). Is there a better approach?
[Note: I have tried to use fraps and other tools to record these sounds. There appears to be issues with some installs of Windoze XP and use of these tools to record sounds. Can't figure out how to record sound. Any help appreciated.]