Yeah I've had something similar actually. Been doing EVP sessions down in my cellar for the past few months and there's a recurring cluster of activity that keeps popping up between 750 and 850Hz on the spectrogram. Not every session but maybe 6 or 7 times now.
What software are you using to analyse? I'm on Audacity mostly, sometimes Sonic Visualiser for the detailed stuff. Would be interesting to compare raw files if you're up for it.
The thing that gets me is the 800Hz range isn't somewhere you'd typically expect bleed from household appliances or traffic noise, so its harder to explain away as contamination. Makes you wonder if theres something about that frequency that's more... receptive I suppose.
Anyone else noticing this? Would love to know if its location dependent or if people all over are picking it up.