Click identification help request

Hi! I’m running Audacity 3.1.3 in Windows 10 Pro.

I’ve been recording off cassette with a Tascam DR-40X onto SD Card, then moving the files to PC and editing in Audacity.
After about a year of using this set-up (100+ recordings), and without ever having noticed the problem before, I’ve recently started running into clicks in the audio.

I’ve attached a few screenshots, wondering if anyone could provide insight into the cause of the clicks based on the waveform/spectrogram? It’s not inherent to the source, because I don’t hear it while monitoring the recording. Is it a problem with the SD card? With the unit itself? With using USB bus power to run the recorder? Any ideas?

Thanks so much!
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That’s possible. I’ve seen similar clicks when recording with my Zoom H2, and changing to a better (faster) SD card fixed it.