USB soundcard

USB soundcard Can be used as a "poor man's 1ch scope" if real scope (or USB-scope, PC-scope) is not available. Also useful for verifying / recording 1 knock-signal (and showing FFT image of it, to see center frequency, and full-spectrum).
To connect to input: 3.5mm jack male, and 10k / 1k voltage divider is recommended (divide by 11) or 9k/1k (divide by 10). Mixing 2 signals is possible, eg. 10k and 20k (the signal via 20k will appear smaller).
Input is only 1 channel (microphone or line-in voltage levels).
Stereo output (do not expect 100dB signal to noise :-)
Audacity is one (opensource and free, for windows or linux) common application to record, view, and edit soundtracks.