Don't know a lot about plumbing and need some advice. Have an upstairs bathroom, that when water is ran through either of the two bathroom sinks and drain, a small drip is heard on the first floor while water is running. Does not happen when toilet, shower, or tub in same bathroom are draining or running water. No obvious leaks in the connections under the sink. Also noticed some infrequent drip sounds when water is not running, if the water pressure to the sink is too high?

Any thoughts?

I don't think you water pressure has anything to do with you noise. Sounds like your sink drain pipe leaks inside the wall where the horizontal pipe does a T into the drain. It may take a long time for this water to show up else where but it will and in the meantine its doing damage to your structure. I'd make an inspectiopn opening on the wall under the sink. It will be easier to haide this repair later.

Thanks for the response. In reading other posts, it sounds like what others have described as PVC expansion/contraction [no noticeable water and sounds like a drip onto metal]. Is that possible, and if so what do you do about it?

Yes it could be expansion/contraction. Try filling a bucket of room temperature water from another part of the house and then pour it moderately down the sink drain. There should be no expansion since the drain won't be hot and your supply pipes were not used. If it is a leak you will heard the drip. No drip, then it probably is expansion.

Again thanks. One last question, if it is in fact expansion, do you need to do anything about it?

Expansion isn't much more than a nuisance to you. Improved clamping of the pipes will decrease it but it isn't easy to get to those pipes at this point.

Hopefully, this will be the last one...

Tried the bucket of water last night, first time heard absolutely nothing. Did it again for verification but used warm (not hot water) and more of it. Heard the dripping sound. Wondered if happened the second time because of the warmer water, so I waited an hour for pipes to cool back down, and did it a third time with room temp water and did not hear a sound.

This has been going on for a little while, and I do not see any water anywhere. The sound is also on the other side of a new furnace we got a year ago, and thinking back it seems like I first started noticing it during the start of the colder weather. I also hear it very sporadically (not with the same rapid cadence as when hot water is draining from the sink), but once every 3 minutes which seems to coincide with when the furnace has fired off.

I am fairly convinced that it is expansion/contraction, but wondering if the rapid cadence while hot water is draining (ie a drip sound every second), which then drops off within a few seconds when the hot water is turned off, and then slowly goes completely away after 15 -20 minutes sounds typical. I should also mention it sounds less like a drip and more like a house creak.

Apologize for continuing to ask questions related to this, but if it is expansion will live with the sound. Just wanting to make sure it is not something more serious.

Thanks for all your help.

Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Regards, Tom