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Re your second question did you try reading up on filters?
https://en.wikipedia.org/wiki/Q_factor
https://en.wikipedia.org/wiki/Digital_biquad_filter

If you are still trying to do this the hard way, by either using or reading source from
https://forum.pjrc.com/threads/25532-ADC-library-update-now-with-support-for-Teensy-3-1
If using Audio...

There really isn't an answer to that one, since without further information everything in a signal is the signal. The noise is in the eye of the beholder so there is no magic noise suppression. From...

If you look at the Audio library
https://www.pjrc.com/teensy/gui/?info=AudioInputAnalog
You can use the Teensy ADC directly, at cost of increased noise and needing input circuitry, which you will...