Those voice recorders, (no I won’t do the Dick-to-phone joke), can suffer from clock noise.
If the recording does suffer from clock noise when you analyse the frequency content of a “silent” part of the recording you will see large spikes occurring at particular frequencies, (often equally spaced e.g. 4000Hz, 8000Hz, 12000Hz). You can use a notch filter to remove these clock noise spikes.
I’ve attached such a notch filter. Put in the frequency of the spike, and use q = 5 to 15 for a big spike and q= 50 to 200 to remove smaller spikes.
Notch filter plugin for Audacity.zip (557 Bytes)