2005 Odyssey Clicking Sound

Everytime the van is started, a ticking/clicking sound can be heard coming from the rear passenger side of the vehicle. It is a rapid ticking sound that lasts about 10 seconds ( or approx 130-150 clicks, yes, I counted) - then the sound disappears. My kids says it sounds like a bomb ticking away =)

I have tried turning the rear A/C off, opening and closing the passenger side sliding door, raising and lowering the windows right after starting the vehicle to see if the clicking would stop then, it doesn't. The clicking stops by itself after approximately 10-15 seconds whether the van is moving or just parked.

Note - I did notice my the van's has dropped to around 13 mpg recently, not sure if it has anything to do with this issue.

HELP!!! the ticking is driving me nuts!!!