Saniflo Gurgle When Flushed

[striker27]

At present I have a Saniflo system connected. Only a toilet and sink. When I flush the toilet I hear the sink gurgle once the system finishes activating. Not sure exactly why. That said I have the Saniflo system vented to the outside as per instructions. The sink is installed directly to the Saniflo tank as per instructions. The main vent stack is roughly 15 feet away from where the Saniflo tank is located. Any ideas on why the sink would be gurgling when the toilet is flushed and how to remedy it??