Well, tested the wires, coil pack, and spark plugs. It's all good.

But, it still rumbles right at start-up. The CEL has went off, but the rumbling still happens.

Car isn't running hot, and is running normal temperature.

Could it still be the Coolant Temp Sensor even if it's not running hot, at all?

It doesn't rumble any other time than besides start-up.