See also in the REF section of the Sportsterpedia:

• Using a Stethoscope or Other to Hunt Down Engine Noises in the REF section of the Sportsterpedia.
  
• Hunting Down Sportster Noises

A stethoscope can be used to find mechanical noises and air leaks.
It is a more sensitive device than a stick or screwdriver.
For that reason, some remove the probe on the end and simply use the hose end for probing.
Then it's less strain on the ears.

Mechanic stethoscope. 1) The black partition at the end of the tape measure is where the diaphragm is. Note the size of the diaphragm.

This can be a spare piece of oil line or even a cut off piece of garden hose.
Hold one end up to your ear and the other end against the testing site.

You can use a screwdriver as a listening device.
Grab a big (biggest you have) screwdriver from your toolbox. ²⁾
Put the metal end on the area you wish to listen to and your ear on the top of the handle.
Noise is transferred through the screwdriver to your ear.