Redline is right on about the noise most often heard. Mine does the same once a year or so. It's possible that something is not right though with that bolt being missing. Did they also put the washer on at the same time. Not worth much and don't loosen the bolts more than just a little when you do. They are made to fit one way. I replaced mine with brass ones (washers that is). There could be a problem with the insert for that bolt. Something made it turn up missing. Check it over good.

Been meaning to mention this. I use a magnifying glass that has a light around it for close work. You can see just about anything with it. Clamps to the bench and has a goose neck thing to move it around with. It will show a crack in something. 25 bucks.