F150 Making odd ticking/knocking noise when idoling.?

I have a 2004 F150. it has about 150,000 miles on it. When i drive for a little while and then come to a stop my RPMs drop to below 500 and then starts to make a load knocking noise. it starts to sound like it is going to die. i just rev the engine and it stops. Does any one know what it could be. if so how much is repair? It is a 5.4L triton. it always makes a ticking Noise when idoling it just turns into the knocking after a while.