My central heat and air is making a loud rumble when turned on. It is the inside unit, not outside the house. The lights flashing say the limit control may be open when these lights are flashing, but I am not sure what that is.
I think it is the bearings, but am unable to oil, and am not very mechanical.
Can someone tell me if a shot of WD-40 may solve this, or do I need a service call... sweating in Texas...

The limit switch is part of the controls for the blower and fuel. It should not directly cause any noise. A shot of WD-40 into the bearing of the blower motor might stop the noise temporarily, at least through the weekend, until you can get a service man.