Hello wheel builders!
I'm looking for suggestions. I've built up a nice set of wheels for a customer using DT Swiss rims, American Classic hubs and DT swiss spokes. The problem is with the rear wheel. Here are the specs:

28 spokes.
drive sides are 14/15/14 and the non-drive are 14/17/14 (Revolutions).
Both sides are laced 2x.
Average spoke tension for the drive side is about 120 kgf.
Spoke tension is very even on each side of the wheel.

When ridden, there is an intermittent "ping" coming from the spokes. It sounds similar to the pops and poings you get from a wheel when it is first ridden. It seems to happen more often when just starting to pedal and seems more frequent at high torque (like when climbing). Has anyone ever seen this? I'm perplexed. Suggestions?