Clunking noise in bathtub after toilet flush

[RBKM]

We are adding a new master bedroom/bathroom. In addition to running new 3/4" pipes to the new bathroom, our plumber upgraded some of our other pipes by replacing our existing 1/2" pipe that led to and from our hot water heater with 3/4" pipe. Since then I've noticed a funny noise in our old downstairs bathroom. After we flush the toilet, at the very end of the tank fill, we hear a double knocking sound that comes from the bathtub that is in that bathroom. Any idea what that might be?
-Rob

[jadnashua]

Some toilet fill valves shut off abruptly. The inertia of the water movement stopping quickly can cause the pipe to move, if it is not anchored properly (this is called water hammer). Most manually opened/closed valves don't operate quickly enough to create water hammer, but things like your dishwasher, icemaker, washing machine, and toilet can. There are two items to look at: are all of the pipes anchored properly, and can you install an arresting device? The pipe moving can eventually wear a hole in it if it rubs on something, not a good thing to happen. A different brand of toilet filler valve may be less prone to cause a water hammer, too. My unprofessional opinion.

[RBKM]

Thanks Jim. What I find curious is that nothing in that bathroom was changed (as far as I know). The only difference might be that the 1/2" supply lines coming into that bathroom now come off 3/4" pipes instead of 1/2."
Rob

[jadnashua]

If, when he replaced the pipes, they were not anchored properly, when the toilet turns off, the pipe can move and bang on something.

[toolaholic]

one pipe is vibrating against another