To vary ISO sensitivity automatically when shooting in dappled sunlight or in other situations in which the subject or background vary in brightness, select On for ISO sensitivity settings > Auto ISO sensitivity control in the photo shooting menu. You'll get optimal exposure without having to adjust shutter speed or aperture, even in mode M.
To limit high sensitivity noise, select a Maximum sensitivity between ISO 200 and Hi 5 (the minimum is the value currently selected for ISO sensitivity unless this is higher than the maximum, in which case Maximum sensitivity will be set to the value currently selected for ISO sensitivity).
To turn auto ISO sensitivity control on or off, press the ISO button and rotate the sub-command dial. The current setting is shown in the viewfinder and top control panel.