Guys please, google it, is a very old well documented issue. Also with af OFF, shutter speed goes down to 2/3 fps in low light.
Do a very simple experiment: remove lens, do not put the lens hole cap over, cover the viewfinder, choose 1/500 and shoot
You will hear 8 fps sound. Put the cap on, cover the viewfinder, no more 8 fps. Disregarding any iso, af, noise setting.