You could try analyzing the sound with Analyze > Plot Spectrum. If you stretch the Plot window out horizontally, you might then see if there are any “aberrant” spikes in the plot. You could then try using the Effect > Notch Filter to remove these particular frequency spikes. This technique (originally explained to me by steve) worked perfectly to eliminate the whine of a heavy duty electric water pump in some recordings I had made at our local water mill.
See here for the original post of mine: https://forum.audacityteam.org/t/another-noise-reduction-problem/24822/1