Today evening several people (ornithologists and non) from the Lab of Ornithology, watched Virginia Rail and chicks at Sapsucker Woods. But looks like Mark Chao and family seemed to have had best view. Many of us thought it was adults who were making this sound. Thanks to Mark's observations and readings, now we know they are chicks!
Here is what Mark wrote to Cayugabirds
"My daughter Francesca and I found five VIRGINIA RAILS (two adults and three chicks) at 6:15 PM on Wednesday in Sapsucker Woods, in the same small cattail patch to which Charles referred in his earlier message.
It took us a full circuit to find them, but in the end, we saw the rails just at the southern edge of the cattail patch, maybe five meters away from the path where we stood. These were life birds for Francesca, and they might as well have been life birds for me, the experience was so thrilling and unforgettable. The adults were brilliantly sharp and colorful, and the chicks tiny, black, fuzzy, admirably confident afoot, and of course utterly cute. Their bills were white with black rings. (We saw both adults at once, and all three chicks at once; therefore I am positive that our count represents the minimum number of rails present.)
This patch is between the Lab of Ornithology and its closest parking lot, bordered on the north by the service driveway and on the south by the paved path that leads from the parking lot to a side door (main staff entrance) of the building. Dave McCartt and my wife Miyoko Chu have told me that people today variously saw the rails from the north and west sides of this patch too. If you go searching, listen for a distinctive loud "pee-eep!" vocalization, which the chicks were voicing to communicate location to their parents, or vice versa. (BNA says it's the chicks.) Note also that there is a culvert under the service driveway, through which the rails could possibly pass under cover on their way (back?) to the Fuller Wetlands."
Here is how they sound. Don't worry about the visual part of the clip just close your eyes and listen. Of course there are lot of disturbances and noises from all around, but pee-eeps are very audible.