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[MiramarTN]

The bathroom vent has one of those insulator pads inserted so no air being pulled through there. I looked under the bathroom sink. The P-Trap has no extensions other than to exit the sink cabinet toward the shower drain.

We have run the sink faucet many times so the sink P-Trap isn't empty.

Based on your ideas, it sounds like it could be vent pipe related.

In addition to the hot and cold water supply lines that come up to feed the bathroom sink faucet there is a cold water line that connects to a black fitting/box that then connects to a white line and exits through the same entrance hole. Could this be related to the vent pipe like SuperD & JamieGeek were suggesting?

Thanks for your suggestions.