Few years ago we were stuck in slow traffic on an interstate - with no A/C in the truck, in July. Windows all down and rolling along more and more slowly. I heard this weird "noisy" noise. Couldn't identify it. We actually thought it might be our truck (and my DH is a mechanic, so not a complete idiot about funny vehicle sounds). We finally identified a swarm of bugs in the trees along the interstate. The noise would speed up as you rolled a little faster and also bounced off the guardrails in a strange way, so you really did think it was coming from your vehicle....
Anyway, we laughed at ourselves once we identified it as bugs. But it took us MILES LOL. But we didn't feel quite as stupid when we saw a big rig ahead of us stop and exit his rig (traffic was stopped at this point) and look all around his truck, under it, etc. to try and identify the noise as well :)
When we had them by our house they sounded like you were living under high tension lines. You could hear them with the doors/windows closed. But it was very area specific.