I have a toilet containing a Gerber 99-646 Pilot Valve. When the toilet is flushed, there is a noise like a fog horn in the pipes and it takes unusually long for the water to refill into the tank. One the water refills, the noise stops. Though all 4 toilets in our condo have the same Pilot Valve, only the one causes this problem.

PS. I noticed that when the noise occurs, if I turn on the cold water in the bathroom sink (same bathroom as toilet), the noise stops.

Several things come to mind.
1) Shut the water off and open up 5the top of the ballcock, (see image) Place a pan over the top and turn the water back on to flush everything out. Reassemble and test,
2) When the noise starts turn the angle stop clockwise until the noise stops. **OR**
3) Replace the ballcock.
'Good luck, Tom