Comment # 18 on bug 780631 from Stefan Seyfried

please raise this with upstream XFCE.
It seems the logout window is a GTK+ error window, which in turn causes the
beep. Or something similar. I could not find the code in xfce4-session that
causes the beep and I would not want to carry a non-upstreamed patch for this
personal preference of some users.

One way to silence this beep, besides blacklisting pcspkr module, is a simple
call to

xset -b

So there are workarounds. I'll close this as wontfix.