Yeah this happens to me constantly and I've never been fully satisfied with the technical explanations people throw at it. The easy answer is RF interference or the recorder's AGC circuit kicking in before it stabilises, and fine, that's probably what it is in most cases. But the timing is what gets me. Not when I'm setting up, not mid-session, specifically in that minute or two right before things start getting active. I've logged it across maybe a dozen sessions in the last two years and the correlation is hard to ignore.
Anyone else actually tracking when in the session these bursts occur? Because anecdotal "yeah it happens" is one thing but if people are seeing a consistent pre-activity pattern that'd be worth documenting properly. Would be good to know what recorders people are using too, some are more prone to this than others in my experience.