RPM WS6

iTrader: (3)

I don't think that piston slap being the cause for a such a noise is "a very rare case", I think it's probably the more common cause although there are certainly other possible causes as well. I would hate for someone to read this thread and get the idea that every slapping LS1 has the issues that yours had, when in fact I've had this noise as well (on a stock example no less) and it absolutely was not the lifter noise you have suggested. Point being, there is definitely not a one-size-fits-all answer for this type of noise and a proper comprehensive diagnosis really needs to be done.