I know this has to be an unique problem bec when I let the Service Writer at my local Honda dealer hear it...he called over a few mechanices and none of them knew what it was. Unfortunately, they were booked that day so I have to take my 2006 EX-L with 56k miles back when they have an opening.
As soon as I turn on the ignition - without the engine firing - there's a very loud and distinct "woodpecker" knocking sound coming directly from the rear of the van on the passenger side - the right rear quarter panel interior. One of the mechanics put his ear right on that panel and said, "Yup - here's where it's coming from - maybe a rear A/C flap or something."
Not real encouraging but I have no doubt they'll find the problem given some time to explore. The sound goes on for about 30-40 seconds whether the engine is fired up or not - all it needs is "key on" to where the dash warning lights come on. Then the sound disappears for as long as the van is running - and returns when it's started up again.
Anybody else out there in Odyssey Land have the same woodpecker noise?