Did you fix this yet? The cheap fix would probably be the oil pump pick up tube O-Ring. Had the same EXACT problem on my buddy's car and we spent over a MONTH trying to figure it out. The X7 lefter is the first to get oil. If the o-ring is bad, then that #7 (driver's side, last cyl back) lifter is not getting sufficient oil = ticking. We EVEN installed brand new LS7 lifters and it still ticked.
Thicker oil pump o-ring GM# 12557752
http://www.ls1tech.com/forums/genera...r-preload.html
Watch his oil pressure boppin around. You can kinda hear it tickin like crazy.