Stink in basement tub / bathroom

I've had tenants in my basement for 3-5 years. House is 30 years old. A stinky smell comes from the tub in basement. It comes and goes but is mostly always there.
Were do I start looking? The fixtures are used so all the traps are filled. Poor workmanship is a possibility but since it hasn't happened for years I tend to think leafs are blocking the pipes or something else.
The smell came _before_ I got the roof worked on, it's still there.
Can I blow (one second plumber) in the tub to push shit out? Any other suggestions?
thanks all cln
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It is a bath tub or wash tub? Is there a toilet there as well? As noted if there is a toilet, the wax ring is first suspect.
For a tub (maybe either type) I would look for a vent problem. A plugged or improperly designed vent system can suck out the water from a trap. Is the tub elevated from the concrete floor?

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Joseph Meehan

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The BATH tub is elevated off the concrete. Yes there is a toilet and a sink in my bathroom. Laundry room is adjacent. or on the opposite side of the wall.
Here's trowing a wrench at things... The previous owner had a cat, I'm surprised it lived as he couldn't take care of himself. BUT. I'm afraid that he might have trown away kitty litter in the toilet. I suspect this would be a complete other problem as the water wouldn't flush as fast or at all. cln
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