This is an excellent question which I have pondered myself. I must say, though, that I am a bit skeptical of the improper valve timing explanation given here. I have heard this very distinctive, off beat syncopation with ex-Milwaukee Road No. 261. Would such a well maintained engine be likely to have a significant valve timing problem?
I would descibe the sound as a pattern of three equally spaced exhaust blasts, followed by a longer space, and then repeating.
RK