Seems like the action may be a bit low for your style of playing. When the strings are close to the fingerboard, they will rattle against it when played hard, as the vibrations are large, causing them to touch the other frets between where you press and the bridge. It may only be a couple of those fretwires - test by playing every note on the neck, ascending and descending, to ascertain if it's worse on particular frets. The neck may not have enough relief in it.
Another cause is not pressing hard enough, or close enough to the fretwire. Pressing on the fret close to the bridge direction makes a better angle for a cleaner sound.
A slight possibility is that the strings are too loose for the open notes - for example if it's downtuned a fair bit, you may need to up the string gauge to regain the tension needed for that particular scale length.