Got a new cassette and chain, freewheel not working correctly
You're on the right track with your Googling; the mechanism which disengages the drive when you pedal backwards (or coast) is plainly not working as it should. That is independent of the newness of the cassette or chain.

If you don't want to get into dismantling the freehub to clean and lube the pawls just yet, you could try your new parts with a known good wheel and see whether the problem goes away. If the problem disappears then you know it is worthwhile to overhaul your freehub.