I read the one on here from a woman who was told that it was something contracting because of the cold weather, hot water etc. She only heard the sound when she used the fixture (flushed the toilet) for a few minutes. The difference is that with mine, the dripping/knocking sound is almost constant. I swear that if it is quiet enough in the house, I can hear the noise, regardless of whether someone has recently used the water, at all times of day. It sounds like it is in the wall that separates my bedroom from the bathroom upstairs. The toilet and sink are both attached to this wall, but not the shower. There are no signs on the wall or ceiling in the living room underneath that there is any moisture anywhere. So, as long as that remains true, I don't need to worry? I can live with the noise as long as nothing is being damaged.

If it is an apartment, call the super... if it's a house, I would definitely open the wall in an area that you can easily repair and is inconspicuous. If you do not see any damage (water stains or weak spots) it MIGHT be OK, but why take the chance?

A utility knife and some joint compound isn't a lot to sacrifice for piece of mind is it?

I am sure a plumber or another expert will comment, but that's my thoughts (had a similar situation in my basement, turned out to be a cracked sewer line... yuk

Calvinjada...

First thing I would do here is to test my toilet and see if it is running water into the toilet bowl at a constant rate... could be the issue right there.

Start by looking into the toilet tank and see if water is right up to the top of the overflow tube in the center of the toilet tank. If so, then water may be feeding in at a dripping rate and that will need to be repaired (let me know if this is the case).

If the water is not up to the top of the overflow tube, take 10-15 drops of a dark food coloring and drip it into the toilet TANK only and then let the toilet sit undisturbed for a couple hours.

Then, go back to the toilet and see if any food coloring showed up in the bowl. If the food coloring does show up in the toilet bowl the flapper will need to be replaced to stop the fill valve from running at a trickle... ;)

Try these things out... let me know what you find, OK?

Finally, no opening the walls for now... take this one step at a time, OK?

Mark

Calvinjada,
I am no plumbing expert but I have heard that noises from pipes can be air in the line. When you turn on a faucet does it spit and spuder at first and then run properly? This could be a sign of air in the line. Good luck.
Rick

Before you open any walls, check out the venting in case there may be a flap which is originating a sound that becomes muted by distance, insulation, etc.

HeavyD... see my comment at your post. Your answer is really not reasonable, especially when the poster mentioned a CONSTANT NOISE, OK?

Hearingthings, you have certainly presented a reasonable answer here, but the fact that they mention that it is "almost constant" makes me think the issue is more as I presented, but again, you never know, do you so thank you for presenting another idea.

Finally guys, look at the dates of the thread in the upper left corner as you are both responding to a thread that is month(s) old and, most likely, calvinjada (1 post) won't be returning anytime soon especially seeing that they didn't respond to the original answers... ;)

Good day!

Mark