usually white bands also called GHOST bands are due to ECL problems or antibody problems, however sometimes I don't worry about them much.
If your white bands are a problem, they are usually due to excessive signal generated by your proteins during ECL. You can try reducing antibody or protein concentrations to get rid of your white bands.
The reason you get white bands is because excessive antibody or protein can cause extremely high levels of localized ECL signal (which is seen usually at single bands).
The result of is rapid, and complete consumption of ECL substrate at this point which basically caused no light production after the completion of this reaction, and so when you go to expose your film in the dark room, you only see white bands when your gel is exposed to film.
Pierce has gone as far as writing up a paper on the topic.
Check this: http://www.piercenet.com/files/AN0011dh5.pdf
Please let us know how you fix this problem (my suggestion is to run a decreased percentage gel and reduce your protein concentration so you can get sharper bands), it is a very interesting question!