I have a 512GB M4 and I can definitely hear a whining/scratching sound coming from the SSD bay.
The whine is pretty apparent for example when I perform a search with the app EasyFind, which can quickly find files/folders on your drive. It only lasts for the duration of the search and I'm not sure how to describe it: it's like a high-pitched screech/whine/scratching that only appears for the duration of the search. I can also hear the noise when I run the BlackMagic benchmark for example, but it's less audible.
I can also hear some faint sound coming from somewhere in my MBP during scrolling on some webpages. That sound is different, though: it resembles a bit the actual scrolling sound of a mouse, except it's barely audible.
At first I thought I have a faulty SSD, but I tried another 512GB Crucial M4 and even a 128GB OWC Mercury Extreme Pro 6G SSD and on each of them there was some sort of barely audible occasional screeching noise/whine coming from the hard drive bay even from the very beginning (when I started installing Mountain Lion from a USB stick).
So this is actually normal??