Upon arrival customer stated they heard a grinding noise over the last roughly week and a half. Checked oil flow and nozzle and both were good. Checked the burner coupling and did not see any issues. Started the unit multiple times and did not hear or see any issues. It is possible the burner motor bearing is starting to go bad and make a noise. It is also possible the oil line was starting to gel slightly because of the cold weather recently. If the oil line begins to gel the oil pump would put more strain on the burner motor and cause it to make noise. If issue persists recommend possibly replacing the burner motor. Performed combustion test and unit is running properly