Trane XE 90 Red LED on front of unit displaying slow blink pattern (continous 1 blink per second). According to chart this error code is "Normal no call for heat". Also the unit is making a low humming / buzzing sound. When heat is turned on the blinking changes to fast blinking pattern, which according to chart is error code for "Normal call for heat". Otherwise all seems normal. Only noticed blinking light because I was doing routine air filter change and happened to see blinking light. Blower runs when a/c is operating. Heat comes on as normal and blower runs just as it should. Unit turns on and off properly when acitvated by thermostat. I have a service contract with a local company, but want to avoid paying fee for sevice call if possible. What sort of problem may have triggered error code to be displayed? And what do I need to do about it? Thanks.