Did you leave it so they could find it? Don't ever settle for "We don't know." Demand a rental, tell them you guys keep it till this is fixed, get the SM involved. Just say, "My new 60k truck is making a totally unacceptable knock noise, YOUR job is to find and repair it...call me when it's done."

Unfortunately you can't pull punches with a dealer, they'll run right over you! Any resistance at all and you go straight to the SM or GM if needed. Service advisors are geared and conditioned to blow people off with noise/rattle concerns as they are time consuming, unprofitable, and no techs want them, so they do their best to shoo them away.