toto gmax, bubbles near stem base - normal?

Have a Toto toilet that has worked flawlessly for 3 years.

Before last week, a flush typically sounded like a big whoosh, then a few seconds (at most) of quiet tank-filling sounds, then a crisp shutoff and nice silence.

Starting last week, although the toilet seems to be functioning perfectly well still, it sounds different. Now, instead of a few seconds of quiet tank-filling sounds, we get say 15-30 seconds of louder tank filling sounds.

Opened up the tank cover and watched the flushing process. Everything looked normal as far as I could tell (am not too familiar with exactly what is supposed to be happening, for this type toilet).

But during the 15-30 seconds of (now noisier) filling, I saw some bubbles arising from down near the base of the stem of the GMax 528.

My question: are the bubbles normal, or could they be a symptom associated with our recent change in flush sounds?

I removed the GMax and disassembled it, rinsed it all off, and reassembled it. Nothing seemed out of the ordinary and the "problem" (longer fill-time; and bubbles unless they are normal) has not gone away.

Any suggestions for the next troubleshooting step to take?

It won't hurt anything, but you can probably restore it to 'normal' if you replace the cap for about $3, available at Lowes or your local plumbing supply store.