A simple stethoscope — yep, just like the doctors on daytime TV carry to make sure you know they’re doctors — can take a lot of the guesswork out of tracking down a noise under the hood. Yeah, you can try the age-old method of placing the blade of a long screwdriver on the suspected source of the racket then pressing the blunt end to the bones in your ear, but the stethoscope does the same thing without you having to stick your head down near a running engine, etc.
You can obtain the medical versions starting at around $15 online — or in a number of stores — but they also make “automotive” versions, which generally offer longer tubes (and lower prices, starting around $5).
We’ve used ours recently in the shop to diagnose bad bearings in an alternator — which we’d feared was actually a water pump going bad. They’re handy.