Retard the timing just a bit. Goes away? It is PING. no problem. If and this is a big if, it is a loose wrist pin, then maybe the "rattle" will still be there. I don't work on that many GMC motors so just guessing on the wrist pin. But timing too far advanced will cause a lot of rattle like noises on these inline engines. Try the timing first. Then get some help from the GMC folks. There are several on here. I am assuming, from your post...it is the stock engine in that 52 GMC in your post? It could very well have a "Chevy" engine in it.
