Dripping noise inside toilet drain pipe

I remodeled a 2nd floor bathroom and yesterday I re-installed the toilet. The toilet drain pipe (which is ABS) goes straight down about 2', then makes a right angle turn. At that right angle, there is a constant sound of dripping water inside the pipe---it's definitely inside the pipe and there's no indication of any water getting out. Also, the dripping seems to continue even after the toilet has not been used for a long period of time (like overnight). I don't see how any problem with the installation could cause such a problem. Any ideas as to what might be going on here? Any suggestions as to how to proceed? Thanks.

Put food coloing in the tank, and see if the flapper is leaking.

HOw are you hearing the sound that you know exactly where it is coming from? IF the tank is leaking the water will continue to drip through the toilet no matter whether it is used or not.

hj said:

HOw are you hearing the sound that you know exactly where it is coming from? IF the tank is leaking the water will continue to drip through the toilet no matter whether it is used or not.

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The right angle I mentioned is exposed in a crawl space, and standing next to that, the dripping is fairly pronounced. Apparently, the drops are falling 2 feet and landing on (the inside of) the ABS pipe. It's kind of surprising that it would be that loud, given that it's inside the pipe, but it is what it is. At a very quiet time, the dripping sound is (barely) discernible inside the house.

I didn't do the food coloring, but I did play with the flapper, getting it more centered over the hole, and that stopped the dripping sound. Unfortunately, that's not going to be a permanent solution, since it tends to pull a bit to the side when flushed, which seems to be the source of the problem.

Btw, this is a very nice high-end Toto toilet (Ultramax MS854114S01) that's only about 2 years old and never gave us any problems. I'm thinking that a very slow leak like this might be a common situation that for a typical toilet drain pipe configuration would go unnoticed. In other words, it might be hard to actually fix. What do you experts think?

It the flapper drops to one side, you might want to see if bending the brass rod helps to drop more centered.

Or it may be fixed by replacing the flapper.
Most dealers like myself carry the $14.99 original flapper,
or Lowes carries the Korky replacement for $9.99

Chemicals inside a tank can shorten the life of rubber parts, and disable any warranties in the tank.

If the flapper does not center itself every time, that usually indicates that it has either deteriorated or it is defective. Replace it and your problem will probably disappear.