What might be the cause of "earthy" smell by toilet?

4 year old home, no signs of moisture, tank and bowl both clean. Other toilet on opposite side of house doesn't have this odor. Flushes ok except sometimes the amount of water remaining in the bowl is exceptionally low and you can hear water draining from sinks in the same bathroom and elsewhere in the house ... show more 4 year old home, no signs of moisture, tank and bowl both clean. Other toilet on opposite side of house doesn't have this odor. Flushes ok except sometimes the amount of water remaining in the bowl is exceptionally low and you can hear water draining from sinks in the same bathroom and elsewhere in the house through this toilet at such times.
Update: What might be the cause of "earthy" smell in toilet?
Update 2: What might be the cause of "earthy" smell in toilet?
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