Often that is part of a delayed ignition of the propane. Not terribly serious to begin with but often gets worse with time. In my limited experience with it, a complete cleaning of the system, combustion chamber and burner included generally improves or eliminates it. Sometime there are problems with the exact proper location of the electronic spark ignition, assuming it has one.
On your old furnace 46 years old now, there was a cast iron main burner which commonly becomes filled with rust which causes delayed ignition. Requires a total removal of the furnace, then the burner to clean it properly.
I play a broken record here, virtually any 46 year old furnace is questionable in safety and probably is best replaced rather than repaired.