I keep hearing this "thwap" noise that I think is a spawner. How can I best locate it?

[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.

Questions:

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.]

It turns out the "thwap" is the "lavapop.ogg" sound. To find the source of the sound, I just had to dig down to the lava lake under neath my spawners. Turning the lava lakes into obsidian (using water) stopped the annoying sound, and now I can go back to my experience grinder in peace. Sadly, there appears to not be a spawner specific sound. I was so excited when I thought there was one. The lavapop.ogg sound got turned on in 1.2.4 (or so).

Do you mean Ambience?

This was apparently added to multiplayer in 1.2.1 however doesn't indicate that a mob spawner is near, rather it indicates that an unlit cave is near.

For ambient sound to play, the game requires a space of at least 1x1x1 blocks positioned 10 - 20 blocks away from the player in any direction, and completely filled with a light level less than eight without any lighting from the sky.

To my knowledge mob spawners don't make any specific sounds which would allow you to identify them from a distance.

Spawners do not make a particular sound. The best you can hope for is "noisy neighbors", which is when you hear the sounds of the mob that the spawner will spawn.

Dig towards the sound. There isn't much difference between a dungeon (that has mob spawners) and an unlit cave with a pile of enemies, though.

In some areas of a desert biome, if you see a large sunken section of sand in a roughly square pattern, that might be a case of where a dungeon is close to the surface. The sandstone holding up the sand was overwritten by the air required as part of the dungeon - and the next block update caused the sand to fill it in. I've found one dungeon this way, so it's rare but possible.