There's a lot of great information about repairing common problems with the Fluidmaster 400A toilet fill valve all over the internet. I haven't seen this particular repair elsewhere, but I have seen people with the same issue, and on most forums, it goes unresolved. People simply replace the fill valve when this happens.
Doing so is unnecessary, and gives Fluidmaster a bad reputation.
The issue is that if you have hard water, are on a well without a good filtration system, or have been using municipal water for many years, your toilet will begin making a loud noise when it's filling the tank. It sounds a bit like a quiet jet engine, and it takes a long time to fill the toilet tank.
These instructions are for an older standard fill valve, not newer eco-friendly, water saving toilets.
TL;DR:
There's nothing wrong with the function of the Fluidmaster 400A valve, but the outlet holes at the bottom of the fill tube (blue area in the diagram below) are simply clogged. This causes reduced water flow, and when the pressure is too high, it's pushed into the overflow hose, then into the overflow tube, causing the jet-like noise.
Vinegar will dissolve these deposits, so dump in several cups of vinegar, and let it rest for at least 20 minutes. Squeeze the overflow hose (red in the diagram below, be sure not to pull it off of the fill valve), and turn the water back on. The extra water pressure should push the loosened debris through the tube, and the water should flow quietly, because it's going down the fill tube, instead of into the overflow.
If it's badly clogged, especially with rust, you need to drain the tank, and pull up on the ring (green in the diagram below), which unlocks the upper half of the tube. Clean the inner fins (brown in the diagram) and everything as carefully as possible, paying special attention to the area around the connection of the two tubes, so you don't wreck the o-ring and end up either buying a new valve, or drinking toilet tank water from your other faucets.
Reassemble, and repeat as necessary.
Rest of the story:
(This seems complicated, but it's only a 5-10 minute procedure, after you've done it once.)
When you flush, the water flows up through the innermost part of the fill tube in the Fluidmaster 400A to the valve assembly, then a portion of it goes into the overflow hose to the overflow tube to refill the toilet bowl. The rest (about 80%) goes back down the outer part of the fill tube, and refills the tank through the holes in the lower portion of the assembly (blue in the diagram below).
Inside this outer portion of the fill tube, there are a few fins about halfway down (brown in the diagram). They most likely exist to strengthen the PVC tube, but they provide an obstacle for debris to slowly build up upon, and depending upon how much debris is in your water system, this build-up can take years, or only a couple of weeks.
To fix the problem, you need to clear the debris. Vinegar is a good solution. It's a weak acid that will remove rust and other build-up, while not harming your toilet.
Flush the tank, then shut off the water at the water valve underneath the tank, when it's halfway full. Add several cups of vinegar, and let it sit for at least 20 minutes. I let mine work for an hour or two, but even though it's a weak acid, you don't want to let it sit there for days. If your water is particularly bad, it can be more useful to do this a couple of times, as the vinegar will only soak into the rust and debris so far.
After it's rested for a while, turn on the water valve under the tank, and let the tank fill completely. Then flush the toilet. For better results, squeeze the overflow hose (red in the diagram), as this will increase the pressure, and push more of the loose debris out of the fill tube. Be careful though, if the blockage is significant or you pull on the overflow hose too much, it may pull off of the fill tube, spraying water everywhere.
If this doesn't restore water flow sufficiently, you'll have to try a more drastic measure. This second procedure carries a bit of risk, so be sure to consider the risk before attempting it.
Turn off the water supply valve under the tank and drain the tank completely, to prevent contamination of your water supply line.
The fill tube is actually two parts, a lower portion that attaches to the bottom of the tank, and the upper portion with the valve. The lower portion has the fins, so to get into the tube to clean them, you have to release the white plastic locking ring (green in the diagram) that's at the bottom of the upper tube.
(Marking where the two tubes join will help you ensure they're fully connected during reassembly.)
Pull the lock ring upward, being careful not to bend or break the fill tube. This takes quite a bit of effort, especially if the lock ring has debris on it. Be ready to go to the hardware store for a replacement if you pull too hard or at the wrong angle.
Once the lock ring slides up the upper fill tube an inch or two, carefully pull the upper portion of the fill tube free from the lower portion. This can be as difficult as removing the lock ring. Fight the tendency to twist it, because afterward you'll have to adjust the air gap line to make sure it's still an inch above the overflow tube. See your valve's instructions for more information (they're available online from Fluidmaster, if you don't have them).
Remove the upper tube, and using a flashlight, examine the lower tube. Remove the debris as best as you can (I use several cotton swabs). Be sure to clean both the fins and the area where the o-ring on the bottom of the upper portion of the fill tube connects, as well as the o-ring itself.
If the o-ring is damaged, you MUST replace the entire assembly, or toilet tank water may seep back into your drinking water.
Better safe than cholera, as they say.
(At this point, I usually turn the water valve under the tank on for a few seconds, to blow out any loose debris, cotton, etc. If you do this, be careful not to let the tank fill up to the o-ring, or you'll contaminate your water supply line. This is entirely optional, but similar to cleaning the valve assembly itself.)
Once everything is as clean as possible, pour a little vinegar into the tube. As long as the supply line is shut off, vinegar won't contaminate your drinking water, and the vinegar is a little extra cleaning beyond what you can reach with the fill tube fins in the way. It will pour out of the fill tube quickly, but while you're reassembling it, the vinegar that remains on the tube will continue to clean it.
Carefully push the tube back together, being cautious with the o-ring. If it goes together too easily (just flops into place loosely), or you feel the ring tear as you push it together, remove it and inspect the ring. If the o-ring is dirty, clean it. If it's damaged, replace the entire valve. Better safe than sorry.
(If I'm sounding a little repetitive about the o-ring, it's because it's extremely important that it seals safely. You don't want to be wearing a colostomy bag the rest of your life because you ignored this detail and poisoned your drinking water. Even worse, you don't want your family blaming you for them having to do the same!)
Once the tube is in place, pushed down where it began (it's a good idea to mark the tube, or at least try to match the marks from the buildup on the outer portion of the two tubes), push the lock-ring back into place, to the bottom of the upper fill tube.
Reassemble the valve cap (if you removed it), check the height of the air gap (twisting it as you separated the tubes can change the height), and open the water valve under the tank. Water should fill the tank much faster, and the noise problem should be gone. Squeeze the overflow hose as before to increase the pressure in the fill tube, just to force out any remaining debris.
I hope this helps and saves you a few dollars!
(Image via USPTO)
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