I listened carefully, and tried to follow the sound. Brown Creepers are usually pretty active, searching for food along tree trunks and branches. Watching for their movement is a good way to find them. Because they call while foraging, the sound usually moves around with them, but this time the call note kept coming from the same location. Eventually I spotted the bird that was calling:
Instead of feeding, this creeper was actively preening. It caught me a little off-guard — I couldn't immediately recall seeing a creeper preening before.
Later I read in the Birds of North America that preening in Brown Creepers has not been described in detail. It was hard to photograph, as the bird was perched in a shadowy place, but here's one image for the record:
I watched this Brown Creeper preen for ~4 minutes. Then it started to search for food along the trunk. It continued to give call notes, apparently keeping in touch with another creeper nearby.
I started wondering about why it might be difficult to observe creepers preening. I'm guessing it's not because they don't preen that often. Do they preen in more out-of-the-way places? Or at times of day when we're less likely to see it? Are they usually still and quiet when preening, and therefore harder to spot while blending in with the bark?
P.S. Click here for some nice pictures of a creeper posted on 5 October 2012.