Been getting this exact thing at Highgate Hospital in Birmingham for the past few months. My Zoom H5 picks up this low rhythmic hum that doesn't appear on the raw waveform when I bring it into Audacity - it only shows up after I run a high-pass filter around 80Hz, which makes no sense technically because that should be removing low frequencies, not revealing them.
What gets me is the timing. It clusters around specific rooms rather than running through the whole building. Room-specific interference on a handheld recorder with no directional mic suggests the source isn't environmental in the conventional sense.
Has anyone cross-referenced their recordings with old hospital floor plans? I've started mapping the interference hotspots against ward designations from the original build records and there might be something there - the strongest signals seem to correlate with former operating theatres. Could be pure coincidence obviously but the pattern is too consistent to ignore.
Would love to compare raw audio files with someone else doing similar work. What kit are you lot using and are you capturing this on condenser or dynamic mics?