IPhone 4 battery charger charges slower than other iPhone

Hello, I am quite fond of iPhones but the iPhone 4 that I am currently using is giving me a rough time. The problem with this phone is that it takes an unusually long time to get fully charged. On average the battery’s charge percentage is at around 50%. I have tried using a variety of chargers including one for iPhone 3 but none of these seem to reduce the amount of time taken to charge. I have the original iPhone 4 charger makes a certain hissing sound when charging. Does anyone know what this implies and what is causing my iPhone to charge slowly?
