Not sure if that's a rod knock; it sounds duller in frequency. A rod knock sounds more metallic in nature. Also, a rod knock will be much more pronounced under sharp engine accelerations. Stab and let off the throttle quickly, and listen for it. Have you revved the engine higher than 2500 RPMs; like up near 4-5000?? Does the noise increase, subside, or stay the same? Is there roughness/vibration in the engine while its running?