Just the last two days whenever the toilet is flushed after the toilet has refilled with water there is a LOUD queer noise in the pipes. It's only just started. There is only one toilet in the house, the water has not been shut off, it's never happened before.

Thank you, C.

The problem originates in the ballcock and is picked up by the pipes on the wall due to a phenomenon know as "sympathetic vibration".
Or lose washer and the angle valve.
1) Next time the noise starts open the angle valve located under the tank counterclockwise until the noise stops, if it stops problem is with the angle valve washer.

2) if still noise close the angle valve take the tank lid off and remove the three screws from the top of the ballcock and pull up the float ball, rod and plunger assembly. Pry out the rubber washer and turn it over. Before you reassemble the ball cock put something over the open ballcock and turns the water on for a few seconds to flush out the supply line. Reassemble and test.

The above is for the ballcock type fill valve, see the images below and tell us which fill you have, Thanks.

John

Also, in addition to John's excellent advice: Close and Open your angle stop (valve under the toilet that supplies it with water )