Did the noise appear as soon as you changed the pads?  If so, look at the brake backplates - the bit of thin metal shielding the inner side of the disc.  They are easily bent when you're working on the brakes and can end up binding on the disc.  If it's not the backplates, take the pads out and start again, double checking everything as you put them back in.  It sounds as if something's been assembled wrongly and is rubbing against the disc.

Tim