I would assume it's all the copper colored metal inside the phone that acts as the antenna. mine is similarly plagued, it seems that after taking one apart, it doesn't quite work as well.
Current devices: Galaxy Note 2 Stock,Nexus 4-stock, galaxy tab 2-7, galaxy note 10.1
Sensation 4G Bruce cm10 jellybean, Asus TF300, Blackberry Playbook, Nexus 1, SE Xperia x1a,HTC Kaiser,Nokia N80,Sony Ericsson W800, Nokia 9500, Motorolla MPX 300 (winmo3), and dozens more. also have an iphone 3g and ipad2 in the family.