I've got a 99 outback with 65,000 miles. I have had it for about 4 months and put 7000 miles on it. I just did a seafoam treatment to it running 1/3 in through the vacuum line that goes to the pcv valve, 1/3 in the oil and 1/3 in the fuel. I put both the oil and fuel in and then put the vacuum line in a bottle and allowed it to directly draw from there. Immediately after I changed the oil with 3 quarts 5w-30 1 quart Lucas oil no slip leak fix heavy weight stuff. Now I have a pretty loud knock at low RPM/idle and Coming down from high rpm. Prior to this the car went through a lot of oil but I kept an eye on it adding as needed. Any thoughts on what this might be?