Hi there,

The rattling sound may be due to the string being loose, or not tight enough wound. Make sure the string is in tune.

If the string has too much slack, it will sag a little bit and when you hit the string, it will touch the frets and give you that rattling noise.

It's odd that it never made this sound before, it's possible also that your guitar's neck might have warped a little bit. This could happen if you leave it in a hot place or in the sun, or any other extreme temperatures.

You may want to try changing the strings and seeing if that helps, or checking the tuning heads, the nut and the bridge to see if anything changed.

I hope that helps!