ProCoach

Yep. Had the SAME issue that I asked my dealer a few times about on my 2016 LT Tahoe. They couldn't hear it but it was plain as day to me. Got worse over time, too. I heard it outside underneath the 2nd row floor, too, as well as the passenger cowl area. Dum-dum is usually used to insulate A/C components and seal openings in the firewall around lines, or at least it used to be. Glad you've found a fix, because this was one of the thigns that drove me to trade for an RST last week...