it's whining noise, not gurgling.
It may be moot, since I will be buying the dual-pump 350 system shortly. I wanted to add a second pump, but the 250 is no longer readily available and you can't add a stand-alone 350 pump to the 200 series controller (per emails with Coral Vue), presumably since the 300 series will be using sine wave technology. And information I've seen online indicates the 350 should be significantly quieter.