External Hard Drive Mistery

I just experienced something interesting. I plugged in my external hard drive to a usb port in the back of my computer, and the hard drive kept making a noise as if it was trying to load iteself, like the kind of noise you hear when your computer is trying to play a scratched CD. Then I unplugged it from that usb port and re-connected it to a port located in front of the PC tower, and the hard drive works fine... interesting..