That was actually a design flaw, but they used it to their advantage. The enemies soon understoood that the ping signaled that the GI's would be reloading and they would then advance, so the GI's would have only half of the men fire and when they were reloading and the enemy would advance the remaining soldiers would fire, thus catching them off guard.
I wouldn't exacty call it a design flaw. It was a considered design decision that had advantages and disadvantages. The garand rifle began development in the early 1920s. At that time, there had been very few successful auto-rifle designs. It was only about 20-30 years removed from the era where all the nations of the world went from single shot rifles like the http://en.wikipedia.org/wiki/Mauser_Model_1871 or http://en.wikipedia.org/wiki/Springfield_Model_1873 to bolt-action magazine fed repeating rifles like the http://en.wikipedia.org/wiki/Gewehr_98, http://en.wikipedia.org/wiki/Mosin%E2%80%93Nagant, http://en.wikipedia.org/wiki/Lee-Enfield and the http://en.wikipedia.org/wiki/M1903_Springfield. These rifles were a gigantic leap forward in technology over the single shot models, what with their ability to http://www.youtube.com/watch?v=YQzJgvj74jE. These rifles were all so damn good, they are still with us today as their features are all staples in modern hunting rifles. There had been some working semi-auto designs: http://en.wikipedia.org/wiki/Mondrag%C3%B3n_rifle. The http://en.wikipedia.org/wiki/Pedersen_device, which effectively converted a M1903 to a 7.65mm assault-rifle. But no army had any semi-auto rifle in general issue to all their infantry.
The Garand was originally chambered in a different round, .276. As for the magazine design, they had a few working magazine designs that had been proven at the time. You have to remember that this was well before the detachable box magazine as we know it today.
1. There was the integral box, which was a magazine held entirely inside the body of the rifle, like the Mauser design, and was usually fed by http://www.aaconsult.com/ammoreview/html/stripper_clips.html. A strip of metal and brass held (usually) 5 bullets together arrainged in a line. The rifle receivers were machined so there was a slot this strip could be inserted into and with a http://www.youtube.com/watch?v=BWsntizb5C0&feature=related off the strip, they woud feed into the internal magazine of the rifle and you throw away the stripper clip. Mosins, Springfields, Mausers all used this method.
2. Some of these designs actually did have the ability for the magainze to detach. An external box magazine would fit up into the rifle action, usually protruding slightly from bottom of the rifle stock (unlike today's long magazines) and lock in place. These were not really designed to be used as replaceable mags. You still fed them from the top with stripper clips and only really took them off to clean the rifle. Rifles with this design included the Enfield.
3. The last type of magazine design was the en-bloc clip design used in the http://en.wikipedia.org/wiki/Steyr-Mannlicher_M1895 designs. It worked almost identically to the above styles, (some were detatchable box, some intergral) but it differed in that it did not use a stripper clip system to feed the rounds into the magazine. The bullets came, instead of on a metal strip, http://en.wikipedia.org/wiki/File:Garand_clip.jpg, double stacked, neatly together. When the rifle is loaded, this entire en-bloc clip is inserted into the magazine well and locked into place by a latch. Just like the internal magazines, the bullets are pushed up by a spring as the rifle action feeds each one into the chamber. The difference is, when you're done shooting your 5 rounds from a Mauser stripper clip fed rifle, there's nothing left in the magazine. With en-bloc clips, after you shoot all the rounds, the clip is still in the gun. The latch that locks the clip into the gun is then activated by the magazine spring follower and http://www.youtube.com/watch?v=WkUjswfbNaw&feature=related. The Mannlicher designs had them pop out the bottom, but this neccessitated a hole in the bottom of the magazine where dirt would get in and foul the weapon.
John Garand decided to use the en-bloc clip because of the quicker reload time. It is much quicker to load an 8 round en-bloc clip into a Garand than to load a stripper clip into a 5 shot Mauser. The Enfield was 10 shots, but you had to use 2 stripper clips to fully load it. The number of working parts in the rifle were no different, nor was it any harder to manufacture, so the quicker, higher volume, (with the original .276 caliber the en-bloc clip was to hold 10 rounds. After the switch back to .30-06, there was only room for 8.) magazine system was chosen. The drawbacks were that you could not top off an en-bloc clip easily and the latch that ejects the clip when empty made a distinctive "ping" when the rifle was emptied. This "ping" has become the Garand's defining characteristic to most people.