sound from sink drain

[tedneeley]

In my guest bathroom (which we hardly ever use) I noticed a pulsating sound coming out of the drain. It's freaky. It's rhythmic, about two 'thuds' per second. I checked under the sink, there's no leak. There are no exhaust fans running in the house. I don't know if there's anything else I need to check. Could there be an internal leak somewhere? It's a hollow sound I dont' know how else to describe it.

[Rich]

My guess is the P-trap has dried out, try running the water in the sink for a minute then listen.

Rich

[greg6755]

Does it make the sound when water is draining? If so it could be a mechanical vent under the sink. Mechanical vents are used when it is not possible to get a pipe to the roof.