Vey simple 30mm cone for plugging into a cheapy stethoscope set bought online (like $5).
The stethoscope comes with a doctors type diaphragm pickup, but to find noisy bearings on my cars accessories (A/C, alternator, P/Steering etc) I found that it's less than good. So I replaced it with a simple cone which works better for this purpose.
Even individual injectors can be heard ticking away individually with it...
I didn't put any science into the design, but it works OK.
Make walls solid, 1mm thick...