Couple of ideas.....
Mine would do this occasionally, wound up being the motor. bushings were bad not letting the motor spin fast enough, not making the sail switch. Easy to diagnose once the squealing started.
Also, if you are not plugged into 120vac, the battery might just be low enough to not let the blower spin fast enough to satisfy the sail switch. Can confirm this by plugging in the unit.
And one other thing is that if the furnace is the first thing you try to light right after you turn on the propane tank, it will take awhile to get the air out of the line and the gas to get to the burner valve. I will turn on the tank, then light the stove top burners and let them burn for a few minutes until all the air is out of the lines, then start the furnace and/or water heater.
Hope this helps a little.