The "warble" is a beat frequency. Any two notes that are only a half step apart will produce a similar beat frequency. This particular one might be more noticeable on your guitar because of intonation and/or differences in timbre that highlight that particular beat frequency.
Actually any two notes played at the same time produce some sort of beat frequency, but it's hard to notice unless the notes are fairly close together. When tuning instruments or voice by ear, one listens for beat frequencies and tries to eliminate them, since two tones that are exactly the same pitch will have essentially no beat frequency.
Notes that are very close together (almost in tune) have such a slow beat frequency that it's hard to hear. Notes that are far enough apart (more than a half step or whole step depending on other factors) can have a beat frequency so fast that it becomes another audible pitch. Those beat frequencies (usually called combinations tones when they become separate pitches) are a part of what makes chords sound the way they do.
If you have an electric guitar, you can usually create many beat frequencies very easily. Dial in a little distortion, if you can, and then play some unison bends. As you bend the unison bend up and down a little, you'll get faster or slower beat frequencies. With enough finger strength and/or fairly light strings, you can do unison bends on the B and high E strings on acoustic guitar as well.
