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?

I might be old school but three cross or even four cross is way more strong.
The spoke ping is caused by micro flex in the hub, in other words the hub wants to spin, the rim doesn't.
The rear wheel doesn't just roll, it has to drive or push the bike forward. A two cross lace pattern has an impossible time tranferring the power from the crank to the chain and hub and lastly to the rim and tire because of micro flex.
A quick solution would be to band together all spoke intersections and silver solder them.
resulting in a much stiffer wheel.
"King"

I'm a fairly old school builder as well. The design of this wheel is based on rear wheels from DT Swiss (RR 1450) and American Classic (420's). Both of these manufacturers build rear wheels using 2x lacing and similar spoke gauges. In fact, this wheelset is sort of a blend between the 2 manufacturers designs.
I'm wondering if the spoke tension on the wheel is low. Like I said, its 120kgf right now. Barnetts suggests that the noise can be caused by low spoke tension, but anything over 120 seems awfully high to me.

Check for a loose spoke.

That was my first thought. I found a few that were a little looser than others, but not significantly. I balanced the tension of the wheel a little better...but no cigar. It still pings a few times per minute .

What about greasing the spokeheads and nipple sockets? I imagine, but do not know, that these might be where the motion that makes the ping is located.

that will help quiet the ping if indeed that is the cause,and just might work in this case, however I feel he needs more tension in the spokes.

Good thought. I did grease the nipple sockets when the wheel was built. It wasn't long ago, so the nipple sockets should be ok. I've been thinking of dripping a little oil on the heads just to see if the noise temporarily goes away.

I've been running ~160 kgf (per Park conversion chart) on my rear driveside spokes for 2 years with no problems, Revolutions 3X, alloy nipples, DT RR 1.1 rims, Hugi hubs. YMMV

you could also put a bit of masking tape where the spokes cross each other to rule that out (or in).
I'm assuming you prestressed the wheels and have ridden them for a while with out improvement?
I built a three cross rear wheel (fixed gear rear wheel) not too long ago and had this noise for a little while but it has disappeared within a hundred miles or so.

Tri-Flow tends to work better on spoke threads and nipples than grease.

Hmmm, very good idea. I think I'll try the masking tape. I suspect the noise is coming from the outermost crossing on the non-drive side. Yes, I've ridden the wheels for about 500 miles (including a double century ).

I had a set of wheels built with old Campy hubs. I had a lot of popping issues that ended up being the cones in the hubs, not the spokes. I know the hub you've built is not free bearings but you could have a defective sealed bearing that is radiating the noise out thru the spokes...