iphone 3g shut of 15 seconds on the apple logo?

[ding d]

I just replaced the iphone 3g back housing cover and put every thing back together but when i turn it on it just show the apple logo for 15 seconds and then shuts off. while is on the logo screen it emit this strange static sounds. oh and the phone must be plug into an outlet to be power on, or else it won't start and it doesn't even work when i plug it in to the computer. please help me if you know how to fix it, i would really be grateful