During filling on warm setting, the hot water gets turned on and off, but it is doing it every few seconds and it is making the water pipes knock. Is there a switch that controls this that can be replaced, or is it all built into the timer control?

You have a problem with the water level switch or the water valves. These are easy to check out and clean. The water level switch needs to be checked with a meter to see if the contacts are opening and closing rapidly, which I have never seen, also, you need to ohm out the water valve solenoids to see if one of the coils has a high resistance. You may or may not be able to see it if it has a break that opens when it gets hot. The other problem could be trash in one of the diaphragms. You could just replace the whole water valve set and if it still does this you will know you have a water level switch problem. Make sure to check the clear plastic hose going to the water level switch. You may want to cut off each end and re-attach it after blowing on it with it out of the machine to check for leaks. Look at these: https://dl.dropboxusercontent.com/u/8010163/Understanding%20automatic%20washer.pdf
https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20Washers.zip
https://dl.dropboxusercontent.com/u/8010163/Maytag%20Washers.zip