You can't get a 2 ohm load from a dual 4 sub, so you're stuck at either 4 or 1 ohm. It sounds like you've got some issues either in the amp or in the signal if it's "knocking and whining". You could also have a short at the sub which will cause all kinds of havoc with the amp and could end up destroying it. Try this: unhook the sub and see if everything seems ok, and if you have any other sub to hook up for a minute try that. That will help narrow down the problems. Beyond that you might need to stop into a shop that has the proper tools and knowledge to test it and get it looked at.