A rattle at a "float" throttle setting neither accelerating or coasting, can be loose rod bearings. If tinkering with the timing doesn't make a noticeable difference, it's probably time to drop the oil pan and check the bearing clearance with Plastigauge. I like to see clearance around .0015" or so. As the clearance approaches the .003" maximum, hot oil pressure will start dropping, and the cruise-speed rattle is pretty common. As long as a slightly worn crankshaft is round and not scored, it's possible to install a .001" or .002" undersize bearing to get the oil clearance back in the ballpark.. Be careful not to get things too tight, as it's not possible to shim-adjust the 54-62 insert bearing rods like the 216's and early 235's.
Jerry
