As stated above the plactic is normal and works just fine.
DO NOT grease a square tapered spindle! As stated also above clean the spindle and the inside of the crank arm and install them as a dry fit ( no grease). You can grease the treads of the crank bolts but do not greae the the taper end of the spindle.
As for for the noise it might sound as if it is coming from the crank/bottom bracket and this could include bad or loose pedals, loose chainring bolts, loose cassette lock ring (not the crank but can sound and feel like the crank), loose derailleur hanger bolt or even dirt between the hanger and the frame. Have you noticed if the noise goes away when you stand up and pedal? You have more load on the cranks at this point but no load on the seat and seatpost. If the noise goes away while standing up check you seatpost (yes, it is not the crank but noise travels through the frame) and see if the seatpost and frame is lubed (do not lube carbon frames or post) and also check to see if the seat is tight to the rails.
Noise will travel in the frame and I have belived it to be in the bottom bracket and it was one of the many listed above.