"New" doesn't necessarily mean "good" where camshafts are concerned, but the fact that it quiets down is a promising sign. The first 20-30 minutes of operation of a new cam and lifters is critical, as that's when they're "wearing in" and establishing a running relationship. If the break-in run isn't done properly, a new cam can, and often does get trashed. As soon as a new engine builds up oil pressure, it needs to be run at 1500-2000 RPM for 20 to 30 minutes. This floods the cam with oil thrown off the connecting rods and allow the cam and lifters to establish the proper wear pattern. Idling a new cam can scuff the tips of the lobes and start them wearing severely.
Chances are that hasn't happened, but I'd suggest using a magnetic-base dial indicator to check the lobe lift on the problem rocker arm and compare it to other valves. Noticeably less lift indicates a worn lobe.
Jerry
