Photo by Dominic Sherony - CC License
I went in, got out my BC Bird Songs CD, and played all the tracks of flycatcher songs until - yes, that's it!! - an olive sided flycatcher! (It was kind of like finally being able to scratch a serious itch!) The song is described as sounding like "quick three beers" - with the emphasis on the 'three'. You can listen to the song at http://www.allaboutbirds.org/guide/Olive-sided_Flycatcher/sounds
At Cornell's All About Birds site I discovered that the olive-sided flycatcher, which comes to BC from South America to breed in the summer, is rated Near-Threatened by the IUCN.
If you live on the north end of the island, I hope that this post will save you the angst of trying to identify the unusual birdsong coming from the forest. Of course, it's possible there are olive-sided flycatchers on the south end too - has anyone heard or seen one??