We also just had one bathroom with a "sewer smell". In our case, when the water was first turned on in the sink, there would be a blast of the foul odor. First I thought it was the water itself, but then I decided to catch the water in a pot - no smell then. But when I poured it down the drain, up came the smell. Aha, perhap ssome bacteria, etc down the drain. So I poured a bit of bleach down the drain and that took care of it. Hope this helps solve your problem. (BTW, if that wasn't the cause, my next step was to pour some bleach down the sink & tub overflow drains.)

Do not be too quick to dismiss your previous plumber for suggesting a AAV (Air Admittance Valve, "studor vent") for your kitchen island. Although the Uniform Plumbing code(UPC) & International Residential Code(IRC)have radically differing views on how a kitchen island vent is to be constructed, nonetheless, both the IRC & UPC do allow an AAV for kitchen island venting. Under the IRC all structures are required to have one "Main Vent",which must run undiminished in size from the structure "Main Drain" through the roof. After that we are permitted to install an unlimited number of AAV's. Under the UPC they do not require a "Main Vent" but all fixture vents are required to be sized by the total DFU (Drainage fixture unit) load of all fixtures served by that vent. Under the UPC a structure is only permitted a maximum of one AAV, with the expressed written consent of the local inspecting authority, and in most cases a kitchen island is the only location where they will grant the exception. Now what is the difference between the kitchen drain and your bathroom drain you ask? When we have a sink on an island it is generally the only fixture attached to that line. As the water drains from the sink it pushes the air ahead of the water in the pipe. Under some circumstances this can result in a slight vacuum in the pipe behind the moving water. This partial vacuum would then result in the standing water in the trap being sucked into the pipe by siphon action and once the water has passed on down the trap is left dry, allowing sewer gas to enter the structure. In this case we install an AAV, which would permit air to enter the pipe behind the liquid flow, thus negating the suction effect. There is a slim chance that you might resolve your problem by installing an AAV on the lavatory drain line, an when you consider the cost of installing an AAV as opposed to opening walls it might be well worth your time to try it, however I suspect you have a different problem in your bathroom. You state that you hear a gurgling sound in the tub or lavatory when another drain is used. It is possible that the water in the gurgling trap is being sucked out, and in that case an AAV might resolve the problem, but I suspect you problem is not a result of negative pressure (the partial vacuum) but rather it result from positive pressure. As the water from one fixture flows through the pipe it pushes the air ahead, causing a slight positive air pressure. Normally that positive pressure would be pushed up and out a roof vent however if your system does not have the required roof vent as one fixture is draining it will push that air up through one of the adjacent fixture drains and out through the trap. Keep in mind that the air in the line contains sewer gas so as that air & sewer gas mix is pushed out you get the odor. The odor may seem greater at some times than others. That does not mean the sewer gas is not leaking because not all the component of sewer gas are odorous. In fact, one of the main gases that comprise sewer gas is totally odorless.

Sometimes there are these type of smells if the pipes haven't been used for awhile, Was the house vacant for an amount of time before you moved in? If this is the case the more regularly you use your water then the smell will get washed away. Otherwise you may have a problem with blocked drains. If this is the case you will probably find out next time you have a party or the like with excess use of the toilets causing them to overflow. If this happens you need a plumber to come out and unblock the drains using an Electric Eel or/and High Water Pressure Jet. If this finds a blockage that that they can't clear then you may have replace/repair your pipes. If so look at the alternative which is to Reline your pipes. This way you can save you thousands of dollars.

I wouls start by putting drain cleaner down the drain often the smell is just youck caugt in the pipes and often this could be it in a old buliding and also the vent pipes could be plugged with leaves or even snow. I would also make sure the P trap has wated in it all the time this creates abarrier to keep sewr smells where they belong. I would start with the drain cleaner and you might want to check to see if your neighbors are having the same issue.