Presenting this as a “mystery sound” is a bit of a trick question, since this is not a sound that anyone would ever hear in the wild. I made this recording when I had the bird in my hand (after catching it as part of a regular bird-banding operation) and held a microphone against its back to record the sound. This is the heartbeat of a Black-capped Chickadee.
The heart rate of birds varies, of course, depending on the species. In general larger species have slower heart rates, and smaller species have faster heart rates. Among the species listed by Calder (1968) the slowest rate was a goose at 113 beats per minute, which is not so different from a human. The fastest measured was a Ruby-throated Hummingbird at 615 beats per minute.
The heart rate of Black-capped Chickadee averages 480 beats per minute, or about 8 beats per second (Calder 1968).
Think about that as you watch the chickadees and other birds coming and going from your bird feeder!
Calder, W. C. 1968. Respiratory and Heart Rates of Birds at Rest. Condor 70:358-365. pdf here: http://elibrary.unm.edu/sora/Condor/files/issues/v070n04/p0358-p0365.pdf