This SAME thing is happening in my EX. I had the motor rebuilt a few weeks back, and for the first 600-700 miles all was fine. Then one afternoon I started backing up and I could hear that thudding/knocking noise. ALMOST sounds like its in the valve case, and I can hear it mostly at about 2300 RPM's. It's a dull thudding like if you knocked on the valve cover really fast with your nuckle. Sound the same as yours?? From the center just above the headers. We checked the oil pan for shavings too... nothing. Checked everything we could think of. I have no codes either.... I had 3 bad motor mounts, and after replacing the upper driver's side the noise has subsided, but it's still there a little - I still have to replace the driver's side lower and I'm hoping that takes care of it. It could be a combination of things...but so far I haven't had any major trouble...please keep us updated as to what you find!