As far as I know, it's the rockers making the mild rattle/lifter tap. Mustangs with the roller rockers were notorious for it as well. Mine did the same thing, just like said above, sometimes it was worse than others.
~Luke Facebook 1969 Cougar - 11.91 - 380HP 2007 GSXR 750 1999 OZ - SOLD - 15.26 NA - 14.71 on 75 shot