It's true traps can generate thuds as well (I'd forgotten about that), although I've never heard it from snares, only lever traps, and I don't think it's birds only (but I can't swear about that). I think the thud in that case is the weight dropping.
Neither a fence nor traps keep robbers or Njerps out (I've had robbers climb fences after first wading through freezingly cold water to bypass the pit traps by the landward fence, and I've had a Njerp systematically disable bear traps (probably a couple of dozen) in a supposedly protective trap line). Annoyingly, the Njerp passed by my stealthed character just a few tiles away without either the trap reset work being interrupted or the dog raising the alarm, presumably because he wasn't actively hostile since he didn't see my character.