Driver seat intermittent vibration bump

[Bruces10]

I have a 2016 Tahoe LTZ which has the driver alert system built into driver seat which vibrates when following too close, not stopping soon enough, etc... Recently, I've noticed the seat "bumps" once on occasion for no reason. I carried it in to the local Chevy dealer....but like a lot of things, the issue never occurred when the technician did his test drive. Just curious if others have had this issue and what is/was the solution? Feels like something rolling around and bumping under the seat.

Thanks!