I have read all these posts with great interest since I have experienced these same noise issues since installing the zipper's 575 gear driven cams last summer. In addition to the cams, I installed S&S roller rockers, S&S gear drive, S&S pushrods, stock oil pump (shimmed). Valve train was noisy, the "clickty clack" sound eminating from the rocker boxes I normally associate with mechanical lifters. My tech (whom I trust) checked after initial break-in and found one of the roller rockers to be defective (i.e. cracked/collapsed cup where the pushrod makes contact with the rocker arm). Unusual problem, but rockers where replaced at no cost to me. However, still noisy, checked again, everything appeared to be normal so I just forgot about it, only noticable under 2000 rpm.
In discussions with Zipper's on the subject, I was told that the roller rockers themselves could be the source of the noise and if it really bothered me, I should go back to the stock rocker arms. However, when they tore down my motor last month for the 120 build I'm doing, my crank was .008 in out of tolerance which, in combination with the gear drive, was the culprit. All that being said, everyone here has touched on valid causes of valve train noise (crank shaft out of tolerance, insufficient clearance in rocker boxes, etc.). The bottom line is that you need to check them all to satisfy yourself that everything is OK. If all checks out, don't worry about it. Just my .02.
In any event, great posts and sharing of information, nice to know I'm not the only one. BTW, I am going to reuse the roller rockers for the 120.