You know, I had a problem just like this a week or two ago. My engine wouldnt turn over. All I heard was a click. I pulled the starter and had it tested at Advance Auto and it passed. Twice. So I spent the WHOLE DAY trying to diagnose it. As it turns out, it WAS the starter. It had a dead spot in the armature. Replaced it, it was perfectly fine. When they tested it, did it spin? Mine did and passed and the second time it didn't and passed. Keep testing it to see if it slows down or stops, or talk to one of the guys at whatever store about it.