About 10 year old High Efficiency Gas Furnace. In Basement of private home. Slow flashing green light (minimal flame detection). Main burner will light after second try by ignitor. Unit will run for 5 minutes then shut down. Seems to be smell of Gas at outside wall exhaust. Unit will restart if I turn the wall mounted thermostat off and on. cleaning flame detector rod did not help. New Furnace filter did not help. exhaust motor and Vacuum switch seem OK. Problem started when outside temperature dropped below 10 degrees. A very slight hum remains "somewhere" after unit turns off. I think this is normal. Any ideas?