Possible knock heard from Little Miami Bike Trail near Ft. Ancient witnessed in Warren County, Ohio

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I was on the Little Miami Bike Trail heading back to Morrow from Oregonia...mile marker 23.5. It was Sunday, April 12, around 5:00 in the afternoon. Since the Covid restrictions were enacted a week or two prior, the trail had became so crowded every day of the week. However a cold front came in and it was a cool, cloudy day and very few people on the trail...maybe 15 to 20 riders encountered from Oregonia to Kings Mill...well, maybe more, but not bad. I'm retired and ride this route nearly day, weather permitting. So, after passing under the former Jeremiah Morrow Bridge (I-71 over Little Miami river) there's a long gentle arc in the trail. I haven't seen a soul since I turned around at Wilmington Rd. I'm cruising, it's nice and quiet, when suddenly I heard a loud knock...so distinct I instantly said in my head...that was a knock! It came from atop the hill to the east. I pondered all the conditions, i.e; kids playing..not!, cold, remote, and they don't play in woods anymore,..adul...