Been noticing something similar actually, though I'm fairly new to EVP work so take this with a grain of salt.
I run a basic setup - Zoom H5 recorder with a couple of XY capsules - and I've been reviewing recordings from a session I did at a local site near Inverness about three weeks back. When I ran the audio through Audacity and applied a spectrum analyser, there was a consistent cluster of anomalous hits sitting right around 90-110Hz. Not wind noise, not handling noise - I ruled those out. The pattern had a different character entirely.
What interests me technically is that 80-120Hz sits in a range that overlaps with infrasound's upper boundary. There's existing research suggesting infrasound around 18-19Hz can cause physiological effects, but I wonder if there's a broader envelope of low-frequency phenomena we're not fully mapping yet.
A few questions worth throwing open:
Are people using consistent noise floor baselines before sessions, or just diving in?, Has anyone cross-referenced these frequency hits with specific architectural features of the location - pipe resonance, structural harmonics, that sort of thing?, Is the 80-120Hz clustering appearing in multiple independent recordings, or single-session anomalies?
Genuinely curious whether this is a wider pattern people are documenting or whether I'm reading too much into a limited data set. Would be useful if people could share what recording equipment and analysis software they're running so we can compare methodology properly.