startup sound

That showed up in MacOS 11...

Turn the Mac startup sound on or off - Apple Support

Try this in Terminal...

How to Disable the Boot Sound (or “Startup Chime”) on a Mac (howtogeek.com)

Have you done a PRAM reset, CMD+Option+p+r...

http://support.apple.com/kb/HT1379

In fact, do 3 in a row, takes a bit of time.

Intel-based Macs: Resetting the System Management Controller (SMC)...

http://support.apple.com/kb/HT3964