Spraying water on a serp belt might not be effective because the single belt will quickly transport the water to all of the pulleys.
I found a mechanic's stethoscope for $13 at my local parts store is incredibly sensitive and can pinpoint noises very well. You also don't have as much danger to self as you do with the old "long screwdriver or combination wrench to the ear" trick.
After you've found your problem, you can use the stethoscope to listen to your whole engine and see what's sounding right and what isn't. Injectors, valves, air pumps, water pumps, anything that moves in the engine has a "normal" sound signature that you can monitor and recognize when it starts to go bad.