It sounds exactly like the tin-horn toot of red breasted nuthatch but 
only in short single beeps instead of the repetitive toot that you 
might expect from a RBNU.  I've also gone through the woodpeckers.

I've been hearing them hear them every morning for the past two weeks 
and I can say with certainty I am not hearing it as part of a mockers 
repertoire. I'm not hitting anything in the Stokes CD's but then, I'm 
running out of patience and getting frustrated. Best guess?
-- 
Melanie Lynch
Annapolis MD