When fill valve in toilet cuts off water there is a loud bang; what can I do to resolve

Sounds like you have "water hammer" going for you. This can be resolved by recharging your air chambers. Let me explain,
We install air chambers on all lavatorys and the kitchen sink to reduce shock, vibration and noise. It could also be that a chunk of solder came loose and got caught in the bend of a 90 degree elbo and every time water rushes past the fitting it disturbs the flow. But I would try recharging first. Has this always been a problem or did it just start? Let me give you instructions on how to recharge your air chambers. Back in the 60's we were required to to put shock absorbers,(air chambers) on all bath lavatorys and kitchen sinks. Over time water and condensate built up in them and they must be recharged. First turn off the house at the main house shut off valve. Then open hot and cold faucets at the farthest bathroom to prevent air lock.(This is important.) You will find under each fixture little brass or chrome valves. These are called angle stops. With a small pail held under the angle stop to catch the run off look under the handle of the stop and you will see a nut with the stem running through it. This is called the bonnet packing nut. Put your wrench on it and back it off counterclockwise. Now open the stop and remove the stem and washer assembly, The stop will begain to drain. When it has quit draining reassemble the stop and move on to the next one. Do this on each one of your angle stops. There will be six of them. When you are done, close off the faucets you have opened, turn on the water to the house, and give yourself a pat on the back. You have just recharged your air chambers. If that don't get it click on back and we will go to step#2. Good luck and thank you for rating my reply. TOM