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!

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