I think the sound the neck makes is also added as ambient sounds.

I've heard it in the Swamps and along the shores of rivers in / and coast of the Black Forrest; while they don't even spawn there.

As for the amount that spawns, considering not every Neck drops a tail, I think the spawning rate is a compensation to that. Personally I don't bother much about it. The moment they're silly enough to slam that head against the head of my mace it's quiet anyway.

Thorin :)