buzzing servos

Hi,
im new to servo use, and I was using a medium servo(http://www.abra-electronics.com/products/ROB%252d10333-Servo-%252d-Medium.html) of operating voltage of4.8-6v. However, it worked without an external power supply. After trying with and without an external power supply, the servo kept on making a buzzing sound after it had moved. BTW, I was using 4 AA batteries. So howcome it buzzes?

my sketch
#include <Servo.h>

Servo servo1;

void setup(){
servo1.attach(9);
}

void loop(){
servo1.write(165); // I changed these numbers constantly from 0-180 deg.
delay(15);
}

I've had a similar (but not necessarily the same 8) ) problem, and increased the delay....

Is the servo under load? Is the servo against an internal stop? Does a longer delay quiten the servo? Servo test code below.

//zoomkat 11-22-12 simple delimited ',' string parse 
//from serial port input (via serial monitor)
//and print result out serial port
//multi servos added 

String readString;
#include <Servo.h> 
Servo myservoa, myservob, myservoc, myservod;  // create servo object to control a servo 

void setup() {
  Serial.begin(9600);

  //myservoa.writeMicroseconds(1500); //set initial servo position if desired

  myservoa.attach(6);  //the pin for the servoa control
  myservob.attach(7);  //the pin for the servob control
  myservoc.attach(8);  //the pin for the servoc control
  myservod.attach(9);  //the pin for the servod control 
  Serial.println("multi-servo-delimit-test-dual-input-11-22-12"); // so I can keep track of what is loaded
}

void loop() {

  //expect single strings like 700a, or 1500c, or 2000d,
  //or like 30c, or 90a, or 180d,
  //or 30c,180b,70a,120d,

  if (Serial.available())  {
    char c = Serial.read();  //gets one byte from serial buffer
    if (c == ',') {
      if (readString.length() >1) {
        Serial.println(readString); //prints string to serial port out

        int n = readString.toInt();  //convert readString into a number

        // auto select appropriate value, copied from someone elses code.
        if(n >= 500)
        {
          Serial.print("writing Microseconds: ");
          Serial.println(n);
          if(readString.indexOf('a') >0) myservoa.writeMicroseconds(n);
          if(readString.indexOf('b') >0) myservob.writeMicroseconds(n);
          if(readString.indexOf('c') >0) myservoc.writeMicroseconds(n);
          if(readString.indexOf('d') >0) myservod.writeMicroseconds(n);
        }
        else
        {   
          Serial.print("writing Angle: ");
          Serial.println(n);
          if(readString.indexOf('a') >0) myservoa.write(n);
          if(readString.indexOf('b') >0) myservob.write(n);
          if(readString.indexOf('c') >0) myservoc.write(n);
          if(readString.indexOf('d') >0) myservod.write(n);
        }
         readString=""; //clears variable for new input
      }
    }  
    else {     
      readString += c; //makes the string readString
    }
  }
}

I have two servos, one without anything, and the other is hooked to a robotic claw. Both of them make noise, but when I increase the delay on 170 degrees to 20, its good. However, at 90 degrees, it does not make a buzzing sound at the delay of 15.
Any suggestions?