2016 Canyon, intermittent noise

[OpsMan]

Trying to trace annoying noise

I drive a 2016 Canyon Crew Cab SLT short box with the 3.6 L . Six months after purchase, the truck developed an intermittent noise (driver side -front end). It sounds like a stray nut or metal bearing rolling back and forth in the vicinity of the driver side door or front suspension zone. Initially I thought it sounded like a dragging brake calliper. The dealer was great and checked the brakes and anything else they thought could be making the noise. Finding nothing, they concluded that a rock may have found its way into the rocker panel or a void space in the front quarter panel and that it would eventually stop. The source was never located but it stopped on its own shortly afterthought dealer's investigation.

The noise has since redeveloped and gotten progressively worse. Currently it is occurring every time I accelerate from a standing stop or come to a stop. It sounds like a mable rolling along a metal channel. As the truck comes to a final halt, the sound becomes more like a marble rolling back and forth, up and down a curved trough. When the truck is stopped or travelling at any speed, the noise disappears. The dealer no longer wants to treat the problem as a vehicle fault (warranty issue) and wants to apply their hourly shop rate to try and track it down. 

I'm looking for ideas that may solve the source of the noise and/or, help to put a stop to what has become a very annoying issue.