Q. There is a horrible sewer odor coming from the bathtub drain in the 20-year-old house we bought last year. This bath is in an addition that was put on about 15 years ago. The odor is intermittent, and some episodes are worse than others. We hear gurgling in the bath drain when the water is turned on in the sink. The septic tank has been pumped and in good condition. Our plumber says they may not have installed a trap. Is there any troubleshooting that can be done, short of tearing out walls to check the plumbing? Thanks for whatever insight you can give us.

PAT SLUZ

A. Most noises you hear from drains usually mean there is a problem with the drain system itself, such as a partial blockage. A 15-year-old addition should have a modern P-trap drain and be vented. Was this work done without a permit? If the bathroom was plumbed by a homeowner intent on cutting corners, he or she may have omitted the trap or the plumbing vent.

I would call in a plumber to diagnose the problem fully, but if you want to do some drain detective work yourself, here are steps that might help:

Check several plumbing fixtures

Try flushing the toilet, running the sink, the tub, and other fixtures in the house to see whether you can duplicate the noise. If the drain used to work but has become slow, I’d suspect a drain or vent blockage.

Look for a P-trap

Is there a recessed light in the ceiling below and near this drain? It may be possible to drop the recessed can down and use a flashlight or an inspection camera to locate and inspect the drain. I use an inspection scope for locating plumbing leaks, evaluating insulation, and looking for framing or structural issues. If there is no recessed light, then drill a nickel-sized hole to insert an inspection scope, which can be purchased at hardware stores.

Locate your plumbing vents

The easiest way is to look in the attic for a plumbing drain line that passes vertically up from the floors and out through the roof. You can also look outside for plumbing vent pipes poking up through the roof. Sometimes these vents get blocked.

Blocked or inadequate vents can produce slow drains as well as noise. If the sink or tub trap is shaped like an “S’’ over on its side, it is probably not vented, and the draining sink is trying to draw air into the drain line from the tub.

Call a plumber to investigate further by visual inspection, drain-pressure tests, or using a drain camera or scope.

Dear Rob

A couple of readers with experience in the industry wrote to me about my column on rubber roofs (“Bounce back from rubber roof problems,’’ Sept. 25). They suggested that the homeowner could change the pitch of his roof for better drainage by adding tapered rubber roof insulation boards. Great idea. Thanks for the tip.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @globeaddress or @robertrobillard.