First pointed out by sagematt in the AB.Net IRC channel, it seems that Naraku no Hana, HiguKai’s OP song, when reversed, sounds rather like Higurashi no Naku Koro ni, the first OP. I shat bricks when I first heard it and it’s really pretty creepy and awesome. The version sage linked was the trailer one, so I did a little messing around with the clean version from the raw, and the results can be downloaded here. Also, if you’re addicted to the song, like I am now after looping it the whole day, here’s a rip for listening convenience.
P.S. Adult Rena looks pretty young to be 38.