1992 Ford Tempo harmonic balancer

I have a 1992 tempo recently I notice that the bolt in my harmonic balancer was totally loose.I tightend it back up but know it keeps backing out and I also have a rattle noise coming form the motor but only when it idles it does not do it when I rev it up or drive it completely goes away gets louder and quiter depending on it mood lol but I am lost at what the problem is the bolt can be tight and it just goes comlpetely loose and the rattle is unknown but the motor still performs perfect and the rattle is not coming from the water pump air cond or alternator etc it is from the motor?/ Help