I have been hearing a 'rattle/knock' type noise from my left manual hub only when locked and not in 4x4.
My question is Am I supposed to lubricate or repack them manual hubs at all?

 

You can pull it and see, it may need re-packing. I have auto hubs, and I've only re-packed when changing bearings, but if the seal is bad or the grease gets too thin, it could leak out. Based on your noise description, you may also want to check the U-joints in the front axle. You may have one or more going bad and thats what's making the noise.