Could be residual PEG from your enrichment protocol.

but it was not PEG precipitated

While not impossible, I don't think it is likely to get phage titers high enough to crystalize. I would first suspect you are having something in the buffer crystalize out (salts etc).

I'm not sure if you are describing the same thing but I have had a white precipitate appear at the bottom of phage preps that are stored for a while and reappears even after filtering. I never definitively identified what it is but I believe it is salts similar to PBS with Mg and Ca that sometimes precipitates out after a while.