Our bathroom sink just recently started to leak in the underneath. I tried tightening up the that attaches the plumbing, and that did not work. When the bowl is full of water, and stopper is closed, no leaks, but I can hear the water trickling. Also, inside the sink, the metal ring around the drain looks like its not really snug fit any more. Thanks for your help!

Hi Husky...

If the metal collar was the issue the leak would show up while the sink was full...

Fill the sink again and then take a flashlight and some toilet paper and find the leak. Here, drain the filled sink and use the flashlight to locate the leak. Use the toilet paper to dab at higher points than the leak and see if the leak is dripping from higher up and just showing up at a lower point.

In this manner you should be able to isolate the leak and tell us where the leak is... see diagram below for proper terminology. Tell us what part number appears to be leaking, OK?

If the leak is not located in the lavatory parts then it is in the pipe and just let us know what material the pipes are made of.

Back to you...

Mark

the metal ring around the drain looks like its not really snug fit any moreThat's called
The drain lip" and if the putty seal's ruptured you've just located your leak. If the seal's ruptured you will see moisture vunder the lavatory above the connecting nut and washer.
To repair, remove the drain lip and run a small rope of plumbers putty or silicon jel around the lip of the drain and reinstall it. (see image)n Good luck, Tom