Hi trickyvicky6 Welcome to Mystic Board
I wonder if you have thought about whether these knocks were just a prank? At night when the air is still sound travels faster and can seem louder, so you wouldn't necessarily be able to recreate the same noise. Also the fact that both of you heard it would imply that it wasn't anything that was paranormal. But it does seem that there must be a reason that this incident has stuck in your mind. Are you happy in your house? Do you want to move? What about a council swop?
There are lots of things you can do to protect yourself from any spirits that might be lurking around the graveyard. Simple things like salt barriers and invocations, paiting runes above your door, or placing crystals around the house and at the gate will all help to imporve the energy. A simple Feng Shui cure is to place small mirrors in the windows facing outwards towards the area of negative energy, ideally these should be silver and six sided but to be honest even silver foil will help lol. Windchimes too will help to ward off negative energies. Stay safe xxxx