I think I commented on this before but Hofer Gurgle Buster coupled to the top of the pipe and possibly a reverse durso at the end where it drains into the sump. I would remove that ball valve or leave it wide open. The other thing I would do is add a ball or gate valve to your return line after the output of the return pump.
Drain noise is caused by too much (sucking/flushing sound) or too little (gurgle/burping) air entering the drain. Ideally the air and water travel down the drain and are separated from each other before the output. This is what a reverse durso does (air goes up and is vented and water goes down to the sump) while the HGB along with the ball valve on the return line allow you to control and fine tune the amount of water going down the drain.
Unless you are going to drill the tank, there really isn't much else I can think of.
HTH.
