To add to what BM says. If you've ruled out leaks and blockages, then it certainly sounds like syphoning.
Wastes are often connected up in strange, inefficient ways. To overcome it, you might need an anti-syphon trap fitted to the basin. Right now you may have a standard bottle trap, or even an old fashioned U-bend called a P-trap.(Shaped like a letter P).
Alternatively, you could fit an inline "air admittance valve". Cheap and easy to fit. It allows air in to avoid syphoning, but doesn't let smells out.