By checking your fingering, you pick one string at a time and see if they all ring well, right? That helps a lot cause sometimes, you just move your finger a little bit without realizing it.

I don't think strumming hard or not has anything to do with it. I might be wrong but for me, if the fingers are at the right place, it will ring true whether or not I strum hard.

When you start to play, does it sound well or only when you get farther into the song?