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If the dealers are stating that the noise is normal then I would ask them to take you for a ride in a new one to prove that the noise is there. If the noise isn't there on a new or newer vehicle then you've proven to them that there is a problem, or if the same noise is actually there then you know you have nothing to worry about on yours. If you've had the vehicle since it was new and this noise is a new development then it's not a normal condition.
If the other shop hears a noise that they don't believe is normal but can't find the source of the noise then using Chassis Ears will help to find it. Chassis Ears is a system that uses microphones that clamp on different locations around the vehicle and they transmit to a box where the user can listen to the different channels to see where noise is the strongest.