When you have no concept of insects being able to make that much noise, the brian just maps it to something else. When you're familiar with them, you can pick out the patterns in the frequencies and the ebb and flow, when you're first exposed to it it's just generic _loud_.
I was staying just outside Toronto (/suburban GTA) this summer; it was a few days before I stopped assuming the sound was construction (sawing). Extraordinary, nothing like it (as far as I've ever known anyway) in the UK. Crickets can be 'loud' (more so in France IME) but these cicadas were in a different league.
