I have to admit, I’ve never heard a “d” sound, though perhaps it’s just some kind of personal difference (rather like the fun with the black-blue/white-gold dress we had a while back). This kinda ties into the fact that the sound just doesn’t exist in English, so your brain just leaps for the nearest equivalent it can think of. Koichi also describes it as part “d” – he says “part R (I’d say around 75%), part L (I’d say about 20%) and strangely enough, part D (I’d give this about 5%).”
For me, I kinda pronounce it with my tongue in the L position, but with the edges touching the roof of my mouth rather than the tip. Which, come to think of it, is probably halfway between the L position and the D position.