The first time John L. Johnson’s artificial hip squeaked, he was bending down to pick up a pine cone in his yard in Thomasville, Ga. Mr. Johnson looked up, expecting to find an animal nearby.

And Edward Heary, an apprentice appraiser in Hatboro, Pa., said clients sometimes look with embarrassment or concern at their floorboards when he walks though their homes.

As all three patients — and hundreds of others — discovered once they pinpointed the source of the noises, they had become guinea pigs in an unfolding medical mystery. Their artificial hips are made of ceramic materials that were promoted as being much more durable than older models. But for reasons not yet fully understood, their hips started to squeak, raising questions about whether the noises herald more serious malfunctions.