Silly season
Hybrids (and electrics) make road/tire noise like any car, and most good gasoline cars barely make any engine noise either. Mary Anne is right, it would be easy to create some background noise coming from the car.
For extra money, you can pick custom sounds like galloping hooves, or the sound of playing cards in bicycle spokes.
Speaking of bicycles, if a bicyclist doesn't make some kind of extra noise on my local hiking trail, I don't hear them coming either. The polite ones give me a short bell ring, or a shout-out.