Hi robski,
I used to make acoustic sensors to detect dry bearings on industry fans (diameter 125cm up to 285cm.)
Bearings make a lot of noise when running dry.
The setup can be made easily:
Use an electret microphone which you glue to the magnet of a defective loudspeaker. Build an AF amplifier directly above the mic with a gain factor of 1,000 (adjustable) and measure the sound volume either with an AC-multimeter or a scope. Remove the fabric on the front end of the microphone since it's glued there to suppress unwanted noise around the mic.
Disregard any clipping of the amplifier if it incorporates AGC (automatic gain control). The LM386 should do that easily.
You'll be able to measure object rpm, loudness (keep the volume control stable after testing.
Use a 3.5 mm mono-plug for the output into ear phones. Using a 3.5mm stereo-plug you might use the third connector for the positive power supply.
You might want to reduce cost using the diaphragm of a pieco-speaker, but you must make sure to glue only at the outer side onto the magnet.
If you drill three threaded sack-holes into the magnet you just need three standoffs to mount the cover of a spray can. That way you keep the interior free of dust/dirt and have a strong handle. The PCB must have three holes to slide over the bolts which must have two nuts on each standoff.
If you want to study similar projects have the German term "Koerperschallmikrofon translated and use that for further search.
Boncuk