Indoor plant soil smells funny.

Not quite sure what to make of it. I'm sure it's not healthy, but the plant doesn't seem to mind. It's a Ficus in a 24+" wide pot. We've had this plant for more than seven years, with a couple transplants during that time.
When I water this plant I get a sickly sweet odor from the soil. It smells very much like I'd expect flat cola sodapop to smell. The small whiff I get from it makes me stop inhaling quickly, so I'm sure that I should be avoiding it.
My problem is that I don't know what's causing it. The soil doesn't seem overly damp. With the depth of the pot, I guess it could be too wet at the bottom, but there is a tray under the pot and the weep holes are open. The Ficus, while not thriving, isn't showing any signs of distress either.
So, any idea what this is and how to stop it? Can I take soil from an indoor plant somewhere and have it analysed at a reasonable cost?
I'm located in Calgary, AB, Canada.
Thanks!
P.s. If there's a better group to ask this in, I'd like to know about it.
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