I think I might be on to something. This might all be due to the toilet wax ring. While it is not leaking water, gas is definately escaping thru it & it may be the cause of everything else...I hope anyway. This issue must stem from the tiling of the bathroom floor. The original flange sat just above floor level -if i recall right. The backerboard and tile of course raised the floor height & I used a ring/collar to make-up for the height difference - I thought anyway. I'm wondering now if perhaps the ring was not thick enough. If so what is the option? Two collars? Are collars available in different thickness?

I think I need to reseat the toilet with a thicker collar in order to get the height just above the new floor level. Am I on the right track here?