If you're experiencing strange (what I'd describe as 'hissing') sounds coming from your router you should try turning off silent boot as it solved the issue I was having with my RB951G-2HnD.
/system routerboard settings set silent-boot=no
The official line from the company is that it can't be resolved which does kinda suck.
If noise from router is as described in forum topic mentioned by your client then that is normal behaviour for some hardware models. What causes it, is gpio pins attached to certain components beeper in this case being triggered at the startup of router by it's hardware before software has taken control of them. This behaviour cannot be corrected unfortunately.