[atv087]

it really could be anything the water pump the power stearing pump the ac pump you would really have to look down with the ac on and see if the ac clutch is binding up if it isnt then your ac pump should be good now when mine went bad it wouldnt knock so that leads me to believe its not the ac pump. Is your engine running hot? or making any funny noises other than the knock? Also if you do find out it is the ac clutch you can change just the clutch with a device called a puller.. (no other way to get it off) You will be able to keep all your freon.