Here's why:
* Secret Headquarters: The house served as the Order's headquarters, and their activities were strictly confidential. Whispering was a way to minimize the risk of being overheard by enemy spies or Voldemort's followers.
* Hidden from Muggles: The house was protected by powerful magic to keep it hidden from Muggles, and even the slightest noise could alert them to its presence.
* Dumbledore's Protection: The house was under the powerful protection of Albus Dumbledore, who had implemented several security measures, including the use of magical creatures and enchantments. Whispering was part of the protocol to maintain this protection.
* Tension and Anxiety: The atmosphere within the Order was often tense and anxious, as they were constantly under threat. Whispering reflected this unease and the need for secrecy.
* Respect for the Black Family: The house was also the ancestral home of the Black family, a family known for its dark magic and harshness. Whispering was a sign of respect for the house and its history.
Overall, the whispering in the hall at Number 12 Grimmauld Place was a combination of necessity, caution, and respect, all contributing to the house's reputation as a place of secrecy and danger.