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Thanks and i assume that Piezo disc microphones, and electret microphones give out a very small signal, so i am going to use  possibly a diff amp to amplify it. This is just to detect if the noise is there or not,  then "inform" the microcontroller.....so any old 'low as possible' component count opamp circuit will do.

Ill just adjust the gain so that only the faulty machine's noise is detectable to the micro that will be reading this (and not the background noise) ....i won't even bother with AC coupling of the signal....just a single supply opamp and shovel the piezo-disc or electret microphone into it......ill probably end up with a rail to rail oscillation at the output of the opamp but thats fine...i can convert that to a form that the micro can read.

If the opamp is giving rail to rail oscillation when the faulty machine is buzzing, and  is lodged at a rail for when the machine is not buzzing, then thats fine.

Crude-as-you-like, cheap-as-it-comes opamp circuit coming up!........with a cheap-as-chips opamp too!

No need for an InAmp here!.....just want to know..."is the machine making the fault noise  or is it quiet?"