Heating Help

Trane propane furnace clicking/popping noise

We have a Trane propane furnace - lit by an ignitor (not a pilot light) - when the furnace starts up and while it is running we hear this loud clicking or popping noise during the whole cycle time.  The heat runs fine, but we should not be hearing the clicking noise the whole time it is running.  Any ideas?

Noise is tough to

diagnose if you can't be there to hear it. Sometimes if the unit is real cold and first fires up it will make expansion noises.