2000 Dodge Intrepid lifters clatter when cold

I have been experiencing this clatter when car is cold after it warms up after 10 - 15 min it does not clatter. I talked to my mechanic and he suggested changing oil and adding Rislone which I did. This worked for about two weeks until yesterday when it started again and yesterday evening ariving at home In let the car idle only to find the oil pressure warning light coming on. The light went off when I idled the car up which leads me to believe this is an oil pump problem. My questions are 1) do you have any other suggestions or areas that I might check 2) is the oil pump accessible without pulling the motor in this model and 3) is it possible that the rislone has thinned the oil and would another oil chang be in order before doing a lot of major enginen work. Thanks for your help, keep up the good work