Update:
The Specialized rep confirmed presence of the noise but had no solution to offer. LBS stumped and frustrated, like me. Still not sure if its coming from the FS or internal cabling but suspect the FS. Love everything else about the bike but the ultra-frequent rattling sound with light-moderate impacts is driving me crazy...
Since I could hear the same sound on all 3 of the 2017 Roubaixs I've ridden (from 2 shops) and a total of 6 mechanics at 2 shops and a Spec rep confirmed the noise I wonder if other riders are overlooking it - it's masked by wind noise above ~10mph so out on the road it can be missed but not so easily in town. I did see a post on one other website where the rider mentioned this unidentified noise was also driving him crazy. Since the other Roubaixs I've ridden also rattled, swapping bikes doesn't offer a solution. I guess I'll have to try to tune it out and accept that a rattle in a $3000 bike is "normal" even though I'm sure Specialized engineers did not intend for it to rattle. I can only hope some Specialized tech or engineer will take this seriously and search for a solution. In the meantime I guess I'll have to accept the "smoother is noisier" reality.