Sorry for the long post... Background: my two cats (boy Francis and girl Snorri) are 8 yrs old, not related to each other, and were adopted together by me when they were approx. 6 & 12 wks old respectively. They were kennelmates so have grown up together in the same house the entire time since then.
On and off for the last 1-2 yrs Snorri will sometimes growl and hiss at Francis. While sometimes it's because he is playing too roughly with her or disturbing her from a favorite spot, many times there is literally no apparent reason--- Francis could be innocently walking past her on the way to somewhere else without engaging her in any way and she will snarl at him as he goes past. Or sometimes She will walk past him and growl at him while he is completely minding his own business.
It rarely erupts into actual fighting but the hissyfits can go on for a while even if I intervene. There's been a couple times where I have had to get up in the middle of the night every few minutes to divert them from their "argument".
They are not free fed and get regular outside activity in our fenced yard daily along with short play sessions in the afternoon and before bed and they are very well disciplined, have more or less regular routines, and are in good health (other than Francis needing to lose a few lbs). A quick water bottle spray distracts them from the moment but then a while later they'll repeat the cycle a few times before the "episode" winds itself down.
It's as if Snorri has multiple personalities because other times the two of them can be curled up in side-by-side pet beds, mutually grooming each other, happily chasing each other around the yard, eating together, etc. And other times it's like she doesn't know who he is. :-( Please help!