I wouldn't lose sleep over this, except the unexpected and fierce sound startles my girlfriend and cat every time.
So I just replaced the refill valve on our toilet (using a kit, so everything from the lock nut to the overflow spout, but not the plunger or handle). It works great. No signs of leaking (at least not on the floor or walls). It does a full refill every time right up to the refill line printed on the back of the tank. The overflow spout is above, not inside, the overflow pipe.
Every so often (maybe once an hour), the toilet shrieks for a half-second, like the float drops just enough to cut on the flow and then rises with just a squirt of water.
My first thought was evaporation, but 1) I'm surprised it's that sensitive and 2) I know our old fill valve didn't do this (at least I don't remember it doing it, and I'm sure both gf and cat would have mentioned it if it did).
The one thing I didn't tweak was the water level screw, because the critical point line and overflow pipe were the specified 1 inch apart (eyeballed) and the water fills to exactly the above-mentioned line.
If it is the water level, is this spontaneous and short burst indicative of needing to lower or raise the water level? And can someone explain why it happens at all? I imagine the water-level adjustment changes where the float "floats" so if the water level is where it should be after a refill, why would it run 30-60 minutes later for a short burst?