Some kind of #mechanical knocking? or bat calls? Bottom line is #mechanical;

#regularity of pulses is puzzling.

the size. Or, maybe just #mechanical noise. But tiny trains are more fun.

Fun #mechanical noise!

I think that's a Bat #searching-call masked slightly by the #mechanical noise and #insect-call

#insect-call which sounds like a rattling noise and, slight #mechanical noise, most notable at the beginning as a 'clunk'