ok so i test drove this montero today the knock comes at about 1000rpm then goes away its seems to run fine but it has the oil light on under 1000 rpm. the idle seems to move around as well, is there an IAC valve on these. the guy said is blew a head gasket and they replaced it. there is no water in the oil. i notice that the lower pulley seems to be moving around a little so maybe the knock is comming from that. Any thoughts?