So what your saying is that you can hear the noise and your brother can hear the noise,,but the dealer cannot hear the noise ?
Either 1 of 3 things exist here. A. You and your brother are just about off your rockers and are shortly headed for the a padded cell.
B. Its just about time to replace the hearing aid batteries that you both have in you matching sets.
C. the dealer needs to replace his hearing aid batteries.
A lot of dealers allow you to drop off your vehicle the night before and this will give the car a chance to sit overnight on the lot and get nice and cold , then you and your brother can go there bright and early in the morning and fire the buggy up and then all three of you can listen to the santa fe's knocking noise.
Then after all of the, oohhs and ahhs and ah hems, the dealer can repair your car and send you happily tooling down the motorway.