I have a 25 year old Trane forced air furnace with central air. Saturday, I lit the pilot light and set the thermostat to heat. As soon as the burners come on, the fan starts - its never done that before. The fan setting on the thermostat is "auto". The fan limit settings are 90 degrees off and 130 degrees on. The remainder of the cycle appears to work normally - the unit runs for a few minutes, the burner shuts off, fan runs for another minute or so then shuts off. Any ideas what could be causing this?

Check the T'stat wiring. I had the same problem when I put in a new T'stat and had to switch two of the wires. Check with the manufacturer for the proper wiring code.

The thermostat has been there for 10 years. I checked the wiring block, all wires are secure. I do recall having to change the backup batteries a couple months back. Is it possible the t'stat needs to be replaced?

Posssible. $25 gets you a new digital; set back type. Just saw a sale at HD todat.