Other sources of double-raps

As I've written previously, Ivory-bill searchers in the recent past have been excited by "tantalizing double-rap" sounds that turned out to be gunshots.

An article in North American Birds (Dec '04 through Feb '05) contains this sentence (page 205):

Other observers have reported double-rap sounds in the Bayou de View, but in some cases, these sounds have been traced to trees knocking together, to Red-bellied Woodpeckers or to Yellow-bellied Sapsuckers.

Note that people have been hearing tantalizing double-raps in the US, over and over, for 60+ years. In zero percent of those cases, an Ivory-bill has subsequently been found in the area. In 100% of those cases, when the source of the sound was positively identified, it was something other than an Ivory-bill.