Got little similiar problem on my p751dm2, its probably coil whining - and yea also have it sometime when I'm on full load while my dc is plugged in, tho there is one funny thing - sometimes it stops and then it starts again but then noise come from different side of pc, although whats really funny that after dc stops whining, 2 other sources starts it... and now to the funnies point - 1 source it's backlight of my keyboard, second source HDD condition led since noise sounds are in hand with it...
Didn't find any proper solution yet, but hope will get it fixed since it's maybe not a minor problem but while doing some quiet internet stuff when fans are almost off this is really noticeable and annoying..