I've had this a couple time over the years. My first thought was the keg I used, but I've ruled that out. I use the same water for every brew, so that isn't the issue. My only suspicion is that it might be related to soaking some of my parts in Oxyclean too long and then perhaps I didn't rinse them as thoroughly as I should have. Perhaps there was some residue that a simple rinse didn't remove.
No aluminum in my system.