I have attached a rough diagram of what I think the venting in the house is like.
There is a problem in that the upstairs toilet is being siphoned when the sewage ejector downstairs goes.
However, the toilet water is not going into the pump, it goes to a septic tank through gravity. So, I can only assume that the toilet and the ejector pump use the same exit pipe otherwise the water would not be siphoned off.
Once the toilet has been siphoned low, then the sink in the upstairs bathroom also starts to gurgle.
Traps on the ground floor seem to be ok even though I don't think they are all vented properly. There could be hidden AAVs but I'm not sure. The only one I can see is hidden in the wall space behind the diswasher.
There is no other main vent exiting the house apart from the one from the sewage tank.
How can I fix the venting in the bathroom so that the toilet and sink do not gurgle? Can I just take a pipe from underneath the sink to outside?
Should the sink and the toilet be on the same waste line?
http://postimg.org/image/9seqyk3sj/
http://postimg.org/image/nkt5u6uk3/
http://postimg.org/image/dm877pl4j/