I agree with road_rascal. I would check the upper air intake first. The top half is comprised of two pieces held in place by two plastic clips that shrink/warp over time and then the two parts rattle making a "knocking" sound. In my case, a 2001 Windstar, I just cleaned the two parts and glued them together with black sensor safe RTV silicone letting it cure for 24 hours before reassembly. I also did the isolator bolts, cleaned the ERG ports, etc. at the same time. No more "knocking" noise ater the fix which was 50,000 miles ago.