**Check the water pressure, then consider a water-hammer arrester near the valve causing the noise.** Draining the plumbing can give temporary relief if you have old-style air chambers: it restores their air cushion, but those chambers can fill with water again. Your two-to-three-day improvement fits that explanation, though it doesn’t confirm it. [Sioux Chief’s air-chamber bulletin](https://siouxchief.com/docs/default-source/technical-documents/other/supply/tech-bulletin---air-chambers-discontinued-3-17)

Here’s a useful DIY sequence:

1. **Check whether slow faucet closure prevents it.** Water hammer occurs when flowing water stops abruptly. If closing the same faucet slowly eliminates the noise, that supports the diagnosis. A brief buzzing or chattering sound as the toilet finishes filling can also come from its fill valve or supply valve. [Fluidmaster’s noise guide](https://www.fluidmasteruk.com/sounds-of-the-cistern-system-what-do-they-mean/)

2. **Measure your household water pressure.** Attach a screw-on pressure gauge to a hose tap supplied by the house plumbing, then read it with all fixtures off. If you have a pressure regulator, use a connection downstream of it. A reading above **80 psi—about 5.5 bar—** calls for investigating the regulator or installing one. Adjust an existing regulator according to its instructions and recheck the gauge. [Watts’ pressure-reduction guide](https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/24275-source/pg-prv-pdf.pdf)

3. **For confirmed hammer, install a sealed water-hammer arrester close to the offending valve.** Look for an ASSE 1010-certified model with connections that match your supply. Retrofit models can fit at a toilet or faucet supply connection; placement near the source matters. Shut off the supply, relieve pressure, follow the fitting instructions, and check carefully for leaks after reopening the water. A sealed piston or diaphragm keeps the cushioning air separated from the water. [Sioux Chief’s arrester explanation](https://www.siouxchief.com/Products/SuperDetails/19)

4. **If the toilet chatters as its valve closes, service or replace the fill valve.** Clean it according to the manufacturer’s instructions first. Also inspect accessible pipes while someone operates the fixture; secure loose supports where pipes visibly move or strike framing.

For your symptoms, I’d start with the pressure measurement and slow-closure test. Those will help you decide whether an arrester or a toilet-valve repair is the next purchase.