A stumper, it seems: 2 plumbers have given up trying to resolve the odor: We bought a house, built in 2001, in May of this year. Upon first seeing the house, I noticed an usual odor in the half bath. Our home inspector suspected that since the house had been vacant for awhile, the water in the sink's P-trap had evaporated, or that the wax ring on the toilet needed replacing. We have done both. Also have: sealed off the AC vent, the stack pipe on the roof (thinking that maybe the vents were too close and causing the odor to come back down), the exhaust fan and the sink drain. Odors continue. We also checked the attic for dead critters. Odor is particularly bad on a humid day, and it smells like a recently cut garlic clove. The septic system passed the test; the master bath whose wall adjoins this bath has no odor. We cut holes in the wall board - nothing found there, or a stronger odor. The house was very seldom lived in during its early years. All ideas are appreciated - thank you!