Just a pointless post, because it interested me :)
 
I looked up "Mechanics Stethoscope" on ebay today, looking for one that looks like what the doctors wear, but not.
 
Turns out the humble stethoscope have moved on, what a surprise.
 
From this at $8. on ebay including postage-
 

 
To this, at $310.
 

 
There is a more reasonably priced one, the cheapest electronic one for $85.
 
Has anybody ever used these, are they worth the extra $80. over the analogue one?

I have used the tried and true "doctors" style stethoscope to pinpoint various mechanical issues over the years in the mechanical trade and wouldn't bother with an electronic one.
The battery never goes flat on the mine and it can sit in the bottom of the tool box for ages and still works when you want it to.
You do need to be careful not to bang the pickup on anything when you have the ear pieces in, its incredibly loud!!

Old school mechanics use simple stuff, when my brother tunes webers he puts a piece of fuel hose into the ram and the other end to his ear to hear what the pulse is doing. He must have better ears than me cos I cant hear much difference to know what adjustments to make!  Len

Then I can not claim to be a "true" doctor......
I have always used a long handled screw driver. The blade end against the noisy bit, the handle cupped in my hand against my ear.
My usual prostheseis is: "thar is a nois 'n tha car, somwhair...."