"Whistles" after BOSE recall action

According to Audi, the cause of the BOSE recall action for "whispering" is the following:
"At the "old" amplifiers, the factory used C-network telephone for vehicles
in addition to:
Shield plate at the top also mounted a shield plate at the bottom. About ET
delivered "new"
Amplifiers have only the upper shielding. The lower one will no longer
==References==

The dealer must not use the lower shielding of the "old" amplifiers
on the "new"
Remodeling amplifiers. He should install it as delivered by ET, since it
other
Complaints will come about by-noise."