Why wont my iPhone charge or be recognised anymore?

hi there after using my iphone for 2 years it all of a sudden started flickering so i went to check the connections on it. after making sure they were fine i reassembled it and it worked brilliant for a few weeks. then it started flickeringagain so i decided to strip it again to make sure the connections were fine, which they were.

however after i reassembled it this time there was a very high frequency buzzing coming from the dock area of the iphone and the apple logo appeared which then fades away. it doesnt seem to charge by any means and it will not be recognised by itunes. every time i put the charger into it i hear a very high pitched buzzing, the apple logo appears, then after 5 seconds the buzzing stops and the logo just fades out to a black screen.

i would appreciate it if somebody could point out whats wrong as i dont want to have to buy every single part of the iphone to try and rectify it.

i have already boughta new dock/charging connector and replaced it but still nothing.

thanks in advance to anyone who can help!