Ultrasonic Sensor - Erratic Pings

Other Hardware Sensors
1

I have an Ultrasonic HC-SR04 sensor on an Arduino Board.

Now, say when nothing is in front of it I keep getting the odd reading, e.g in this case 6cm ping... so it may go like this:

Ping: 0cm
Ping: 6cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm
Ping: 0cm

I am using the NewPing library.
I also tried using the ping.median to see if it would eliminate the odd 6cm ping.
I changed the sensor - same model.
I changed the location (in case it was dust particles / electrical interference)
I've now run out of ideas?
Any help appreciated.

// ---------------------------------------------------------------------------
// Example NewPing library sketch that does a ping about 20 times per second.
// ---------------------------------------------------------------------------

#include <NewPing.h>

#define TRIGGER_PIN  8  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     9  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 200 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

void setup() {
  Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
}

void loop() {
  delay(50);                      // Wait 50ms between pings (about 20 pings/sec). 29ms should be the shortest delay between pings.
//  unsigned int uS = sonar.ping(); // Send ping, get ping time in microseconds (uS).
   unsigned int uS = sonar.ping_median(10);
  Serial.print("Ping: ");
  Serial.print(uS / US_ROUNDTRIP_CM); // Convert ping time to distance in cm and print result (0 = outside set distance range)
  Serial.println("cm");
}