I went to Audrey and Eli’s school last week (held outside in the playground) and the noise of the chirruping from the park next door to the school was so loud that you couldn’t hear the teachers speaking.
They’re big insects with large eyes. I imagine they’re pretty harmless and the birds feast on them. They must migrate somewhere I guess as you don’t see or hear them all year round. Here’s one I spotted on my walk home the other day.