I know this sounds like a timing issue but I have set the timing 6000 different ways and it still rattles under load at low rpms. it is a 88 bronco with the 302ho. it runs good other than the rattle, could use a little more power but thats a different story
It could be an exhaust leak,eg:cracked exhaust manifold,broken AIR check valve behind the upper intake on the pipe that is bolted to the back of the cylinder heads.If you can hear the noise under load(in gear with the brakes on and increasing the rpm slightly),it is probably an exhaust leak.
Carb or EFI? If EFI don't forget to remove the spout before you check/set timing. Make sure you don't have a vacuum leak somewhere. I just squirt my engine while it is running with simple green. the liquid will block a vacuum leak and you will notice the idle improvement. Besides....keeps the mother clean to and is nonflammable. :toothless