squealing/whistling audio

Sometime in the last couple of months an old problem has resurfaced on
HEAD, a sort of squealing/whistling sound in the audio, even without
anything playing. The sound is similar to the wind whistling through
something.

Before I blindly go off on a bisecting spree, does anyone have a
suggestion as to where I might look?

Doug

-- 

    I am only one, but I am one.  I cannot do everything, but I can do
    something.  And I will not let what I cannot do interfere with what
    I can do.
			-- Edward Everett Hale, (1822 - 1909)