nick is bang on seen it regular on cambelts its hitting the thermostat housing,when the belt tensioner is turned the correct direction to tighten you will have clearance between the stat housing and the belt .but there is another reason for a track in the middle of the belt,and if you look close you will see that it appears to be raised in the center,this is a kind of dirt and belt particles track caused by the split in surface of the belt tensioner pully and is effectivlly a part of the back of the belt that is never i contact with any surface