Our plumbing contractor friend is one of the many old-timers that like that 400A fluidmaster, and they never miss a chance to say so. Kinda like someone saying, "There's nothing better than a good old fashioned pencil and paper!" But I digress because our friend is kind of missing the big picture about your problem.
There's some teaching on here about water pressure and expansion tanks that might be worth reading, and which I only summarize below, but I think it's the key to your issue, as described below.
You report that the noise goes away when you release pressure in the system by flushing, then returns later. If you didn't have the hum before, then the new stuff you installed is a logical potential source, but maybe not in the way you are thinking. It's not that the new stuff isn't working; it's that it IS working. Here's why:
When water pressure in a home is too high but not high enough to trip, say, the T&P valve on the water heater, the high pressure will find its way out of the system through the least-resistive valves. Often this is an outdoor hose bib or the like that just dribbles a little all day and nobody notices. Very often, it's the toilet fill valve; the pressure just pushes a dribble past it.
And indeed, you mention that the fill valves were running a little. You may be surprised to know that when the fill valve is running, a defective fill valve usually isn't the cause; usually the fill valves are fine, and they are operating properly to refill the toilet because the flapper is failing and letting a small amount of water out of the tank, which the fill valves are duly replenishing. So, often on here, when people say their fill valve needs to be replaced because their toilet is running, we tell them to check the flapper, and, sure enough, when they replace the flapper their "defective" fill valve turns out not to be defective. But that's not the case for you. You DID have a runny fill valve that had nothing to do with the flapper, because when you replaced the valve you solved the problem. If you had a defective flapper like most people do, you would have been telling us that you replaced the fill valve and it DIDN'T solve the problem. But it did. So the fill valves were, in fact, leaking.
The most likely cause of two fill valves simultaneously leaking in a house is higher-than-optimal water pressure pushing around their seals.
You replaced both seals, and they are plainly holding. So...the water is now finding another valve to squeeze out around, and when it does so, wherever it is, it is causing that hum. (This is further shown by the fact that when you replaced only the first valve, no hum. When you replaced the second one, and the toilet fill valves were no longer the weakest seals in the system, the water found somewhere else to start pushing around, leading to the hum.)
One of two possibilities: First, your city pressure is high, and the pressure reducing valve coming into the house is either failing or improperly-set, so you have too much pressure. Second, and probably more likely, you have a "closed" system (either owing to a pressure reducing valve with a check valve, or due to a backflow preventer where the water comes into your home), which is more and more the norm. (Cities are trying to prevent any contamination that occurs of the water in your home system from becoming contamination of the City's system, so they require a check valve or backflow preventer at the water meter.) When your water heater fires up to heat water, the water expands. In the old days, that expansion would be distributed over the entire city water system, because your house water had an open connection to the city water main. In a system with a backflow preventer or check valve, that expansion of water takes place solely within your home system. Because pipes don't expand much, and water doesn't compress well, that expansion takes place in a relatively-small space, leading to a spike in the water pressure (on the hot and cold sides, both). We could be talking as much as 20 psi or more. That's why most new water heater installations have basketball-sized expansion tanks, which are filled with air at a certain pressure and absorb the expanding water from water heating, maintaining a constant pressure. Sometimes, even if you have these, the bladder fails or it needs to be recharged or becomes waterlogged, and its effectiveness fails.
Anyway, to test my theory, get yourself a little cheapo water pressure meter at the hardware store, preferably one with a little telltale that lets you know the max. Hook it up. Take a reading. Flush. When the fill valve shuts off, take a reading. Then, wait until you hear the humming and check the reading. I'm guessing you will find it to be too high, and you can address the cause of the overpressure, whether it's due to your pressure reducing valve near the meter being improperly set (or nonexistent) or due to the absence of an expansion tank (or one that is becoming defective). (BTW, if the pressure after flushing returns immediately to the too-high-level at which the pipes are humming, it's more likely your city water pressure/failed pressure reducing valve. If the pressure stays pretty low, then spikes up when you hear your water heater running, then you know that it's likely thermal expansion.)