Whatbird.com

I am in So. CA (suburban Riverside County). This bird(s) just appeared this week. It makes the same short whistle sound, that repeats every 2 seconds or so. This goes on for hours. Searched my guides and the Net but can't figure out what it is. Any ideas?

Please help a beginner - I can't get this song out of my head without learning what it is! I heard a very distinctive but unidentified bird call 2 weeks ago at Point Reyes National Seashore, near the Fivebrooks trailhead. The call was coming from near the top of trees in mixed forest (willow, spruce, bay, etc.) between a small pond and a ...