Is the problem coming from one drain inside the house?
You need to know if the problem is an isolated case. Chances are that if gurgling is coming from all the drains inside your house, the problem is external.
If the P-trap is damaged, it can create a vacuum or cause the water to drain slowly. The P-trap is an essential design element in the drainage pipe in that it prevents sewer gases from coming back into your house by creating a water seal.
When you have tried everything to no avail, perhaps the best approach would be to call a professional. These professionals have the right tools, experience, and knowledge of how to deal with such issues.