Shower drain gurgles when toilet flushed

Hi folks-
A strange thing has been happening for a while, hasn't gone away on its own, and it's time to investigate. This started happening last winter.
My upstairs bathroom has a toilet, sink, and shower. When I flush the toilet, the shower drain gurgles. No toilet water backs up into the shower, and in fact as near as I can tell, the gurgling seems to be the result of a _suction_ on drain pipe, rather than a backup.
The house has been prone to roots blocking the outgoing sewer drain in the past, but no problems with this recently. There's no reason to suspect any sewer line blockage at this time.
My thought was maybe there is a blockage in the vent stack somewhere. In this scenario, when the toilet flushes, the rushing water causes a sympathetic suction on the associated drain pipes, including the shower drain. No gurgles in the sink drain, but it's farther away and maybe on a different vent.
I guess I could call a plumber to investigate, but that gets expensive and the noise is only mildly irritating. Anything I can check on my own?
Thanks in advance!
Marc
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Do you use the shower? You will get the gurgling sound from a dry trap because the shower/tub is never used.
I have run into this more than a couple of times because people will use the toilet in the guest bathroom but never the shower.
HTH
rik
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snipped-for-privacy@verizon.net says...

The wife and I are squeaky clean; shower used several times per day.
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MAG wrote:

I agree; you're on the right track. Shine a bright light down the shower drain so you can watch the water level when the toilet flushes. That will tell you if it is indeed "suction".
Venting a shower drain is often problematic in that the drain has to run in a confined joist space. The vent take-off is often, at best, at a 45 degree angle. If they laid the take-off horizontal, great clumps of hair can get in the vent.
You can't snake that unless you can access that portion of the vent system from above.
Then again, perhaps the vent stack is blocked...
Jim
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(snip)
(snip)
(snip)
Hmmm... a hair plug is definitely a reasonable possibility. Any suggestions for clearing it in the event that I can't get at it from the roof vent stack? Leave some liquid plumber in the drains for a weekend? Plunging?
Marc
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MAG wrote: <SNIP>

Plunging will tend to force any clog *up* the vent. Hmmm. Maybe a shop vac sucking on the drain while the roof vent is closed off? I have a hunch it will take making access to that particular vent line...
Jim
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In a few months I'll be doing a kitchen remodel. The shower, and presumably the vent for the shower drain, are on the other side of that wall, which will be temporarily losing its drywall.
If that gives me access to the vent, is it OK to poke a hole in the vent stack, snake down the hole to clear the obstruction, then use a pipe clamp to patch over the hole? Or should I plan on cutting out a foot or so of vent pipe, snaking out the trap, then replacing the pipe?
Marc
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MAG wrote:

See if you can just cut the pipe, move it aside to get the snake in and then re-connect with a "No-Hub" coupling. Jim
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