Hi @gabbivanbaalen!
I often have my camera set to AUTO because the manual modes often make my photos ”grainy”, though I can’t figure out why.
What you said makes me wonder… is it a really high ISO by any chance that makes your photos grainy when shooting in Manual (M) mode??
Other than that, maybe there’s another setting that is making your photos grainy when you try shooing in Manual mode.
My advice is to try putting the camera in Manual mode for a little, and then try tweaking with the different settings to see if you can find out what the problem is. (There are probably some shortcut buttons on your camera that you can use so that you can take control of the ISO or aperture while your camera is in Manual mode.) Hope it all makes sense! 🙂