ITB's and foam filters?

[falcor]

I was at a local car show last week with my ITB'd 914. One guy was very interested in my engine modifications so I popped off the drivers side RamAir foam filter to show him the ITB setup a little better. Then I noticed the "roof" of the filter being slightly zinged, can a backfire cause this? It's happned once or twice but I didnt think much of it when it did. I'll post up some later when I get some garage time.

[falcor]

This is how it looked, its only on cylinder 1 and 2, nothing on 3 and 4. Images are clickable to increase the size.