My 2003 1500HD, which is new to me has a pretty good rattle under acceleration consitently at 2300-2500 rpm, and in any gear. Seems to be more from the right side. Defintiely not kncking or pinging, just did a Blackbear scan cable tune and nothing showed up. Still does it after the tune. Anyone with a similar issue and what was the fix?? Thanks

Well your air conditioner pump is on the passengers side, it makes a slight noise but not usually a rattle.

My first thought when I read your post was a broken bolt on the exhaust manifold. It is usually the drivers side for that, but maybe give them a check. The 6.0L has issues with them breaking, but its usually the back one on the drivers side.

Check both belt tensioners especially the AC tensioner. It's constantly soaked by water. Mine was making a horrible rattle. Replace both the tensioner and idler if that is the source of the noise.

My alternator rattles under load and when its cold. Its on the wrong side but you could check there too.