One good diagnostic tool for locating sounds is a mechanic's stethoscope. A long screwdriver can substitute but doesn't block out surrounding noise like a stethoscope does. It won't tell you specifically WHAT the noise is but it'll let you know WHERE it's coming from to help narrow the search.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.