When i play guitar, sometimes some strings ring out even when i dont touch them.?

I have been playing guitar for near on 4.5 years and there is one tiny thing that still bugs me. I can hear my strings ring out even when im not touching them. i have looked very carefully and im not touching, strumming or having any contact with the strings that are ringing out. i have had this problem on multiple guitars and amps so its not that, so i can only assume its me. whats goin' on!?