Our unit was installed in 08, it has all original parts, and has worked fine. However, during the last few days we've noticed it is slow in filling and makes a longish "burping" sound, and then a kind of hiss just as it shuts off. Obviously, these are highly subjective terms, but it's the best I can do.
I've gone through most of the posts, and can't find anything that seems to fit. The tank fills to about an inch below the top of the flush valve. The valve itself feels properly seated, though I haven't checked for cracks. There is no sound of leaking water, however. I've pressed down on the flapper to try to make sure there's a good seal while the fill is going on. There's slack in the chain above the "float," but it is taut below.
As for the sound, I saw that you used the term "water hammer" to describe another member's problem. Regarding pressure on the water heater, etc., we did drain the water heater and shut off the water supply to the house for three months during the winter.. Activated everything six weeks ago.
Most likely the problem is with the fill valve in the toilet. You might turn the water all the way off and all the way on there at the wall by the toilet just to make sure that there isn't anything sticking that is causing this (i.e. gunk). Then, it's most likely gunk in the fill valve that is causing your issue, based on the fact that you had drained and refilled your water system.
Easy to fix. Here are the service instructions for the fill valve. It's easier than it looks to service it. On the service instructions in the link below, there are pictures of the Type A and Type B valve, so you can figure out which one came in your toilet. The Type A valve is made by Voreto in China to a Toto design. The Type B valve is made by Korky in the US. They are used by Toto interchangeably. If you need a new fill valve, you can just pick up the Korky 528MP Maxperformance fill valve at any Lowe's and many local hardware stores. (HD doesn't carry it, which is odd, since it's the #1-selling Korky product at Lowe's.) It's the one with the silver cap. Don't get the white-cap one because your toilet requires more refill water than the white one will provide. You just run the 528MP with the little refill adjuster wide open in the old Ultramax and you're perfect. (You'll see what I mean if you get to that point.) Your toilet could also take the Korky 528T "Fits Toto" fill valve, which now seems mostly to be sold at local hardware stores.
Further, if you have a Type B valve, here is the video from Korky on how to service it. The Toto piece is a little more comprehensive, but the video is helpful in seeing how it's done:
If servicing the valve doesn't fix the problem, and you have the Korky Type B valve, you can just replace that little cap with a new one from Home Depot for like $4. It's Korky Part No. R528. If there are big indentations in the rubber when you take the cap off to rinse it and it's not smooth like in the video, it's probably time to replace it.
NOTE: Amazingly, and reprehensibly, Lowe's seems to have stopped carrying the R528 Cap, but HD still has it, as do many local hardware stores and plumbing supply places, But really, now, Lowe's. That sucks.
This is a fun, easy project that you really can't do wrong.
If you do end up replacing the fill valve, here's a video by Korky on how to do it:
Come back with any questions, and let us know how this turns out for you!