Making note of interval timings with a stopwatch can show whether it really is random.
Disconnect all batteries & shore power
and if it stops check individual items. Bilge
pumps, level switches, gas detector, log impeller, depth
transducer, ultrasonic antifouler - everything.
If you're moving or the water's moving maybe something's caught up on the hull
and fluttering in the flow?
Something like a marble rolling around in the bilge
I didn't even know there were noisy little fishies
Except the ones sizzling in my frying pan.