In addition these spectral characteristics for clicks/pops, in your audio, localize some of your pop/crackle sounds by ear and scrub over them in the timeline, while looking at the spectrogram. You'll soon be able to pick out the anomalies visually.
You may also need to tweak your "spectrogram" view settings to show these artifacts more clearly.
Audio sampled at 44.1 kHz (like normal audio CDs) can in theory contain content up to the Nyquist limit of 22.05kHz. However, you need a filter to remove all content above that limit, otherwise it folds back into the hearable range: you hear this as aliasing.
A perfect, theoretical filter would remove all content above 22.05kHz and leave everything else ...
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I think you might be placing too much importance on what this can do for you. Looking at what you have there, a lot of the noise floor is just broadband noise, I assume that came from the transfer medium. Clicks will show up as vertical lines when zoomed in, but pops and crackles are less easy to spot - especially amongst all that noise.
You probably already thought of this, but if there is a common sound/signal frequency wise, simply taking the average of all the FFTs will attenuate the uncommon parts, while the common part remains roughly the same level.
The spectrum plot below illustrates that - in the center section you see an average of that period. The lines (the common signal) ...
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