I don't have the HDD Acoustic setting in BIOS either on my 755 SFF with A19 BIOS.  In looking at this further (which I somehow didn't think to do until now), release notes for the 755 BIOS version A16 says: "Removed HDD acoustic support".  So, in order to get your Acoustic setting back, you will need flash back to A15.  In so doing, however, you will lose the following fixes/enhancements from the following BIOS updates: A20:1.Update MEFW 3.2.60.10602.Add Signed Firmware Update Feature A19:Added new master password algorithm support A17:1.Updated PSA diagnostic 2.Updated ImageServer Option ROM 3.Updated Intel TXT module 4.Enhanced the method of specific smart card support. 5.Added back 512MB memory configurations support which was removed in A16 BIOS A16:1.Removed HDD acoustic support 2.Added remote TPM activation support 3.Updated Intel ME firmware 4.Updated ImageServer Option ROM 5.Secure MEBx6.Removed support for 512MB memory configuration  
Hi folks,  After BIOS update A20 version for my Optiplex 755 SFF there is no HDD Acoustic setting in BIOS! I couldn't change this settings over Dell Client Configuration Utility...Any idea?
BIOS update