I have been experiencing a water dripping noise (or kind of a water dripping on something) from my wash basin on the second floor bathrooms. The noise is when the water is drained from the wash basin. To test it out I filled a bucket of water (from 1st floor) and drained it in the wash basin in Master Bedroom and second bucket in the common bathroom. As soon as I drained the water I could here dripping water (fast initially) and then slowing down slowly. The noise is kind of water dropping on something.

I have noticed this for last six months. I have not seen any leakage sign on the 1st floor ceiling, no signs of water at all anywhere. I DO NOT get same syndromes from the first floor wash basin and kitchen sink. No sign signs of any leak in the basement either.

The house is 28 years old and I am in North East (NJ). I am getting concerned that this may be warning before some big trouble.

Any thoughts on what might be the issue?