Charger Leads to "Whistle"

Hello,
I have a Dell Inspiron 6000, I've been having trouble with the AC-Adapter and getting a good charge. I usually have to move and twist the adapter until it starts getting a good charge. Recently, my computer wasn't able to charge at all and shut down completely. So I had to purchase another adapter; I did and it works most of the time, but I again have to twist it until it starts to get a good charge. Also, a "whistling" sound has started happening; when I unplug the charger, it goes away, but when I plug it in, it's there again. What is wrong with my computer?

Thank you in advance for your help.