Wonder if an ordinary stethoscope would work thru egg shells. They used to be pretty cheap on pet supply(having medications etc.) on line. Sometimes nursing supply sites carry them. You might even score one from a doctor or nurse, when they replace their older equipment.
I had cheap ones I used for years when I raised dogs but, then started tinkering around using it on myself. My doctor made me give it up because I was "finding" all kinds of scary noises using it. Hey a little hypocondria is good for ya.