Extensive flow field studies on scaled F-15 models show velocity fluctuations that amplify and become more narrow-band at down steam locations.  At the top of the vertical tail, nearly periodic fluctuations are the dominant flow feature.  This is has been shown to be true with and without tails present.  The frequency corresponding to this spectral peak is seen to vary with angle of attack and freestream velocity.  This implies that at certain flight conditions, the velocity fluctuations will tune to a tail structural mode; thus causing tail buffet.