Humming/Droning noise mistaken for road noise at first

can you safely do this test you mention whilst on the freeway rolling around 65mph?
my mother has an '11 4wd single cab with the AT (3speed with D) and the 4cyl engine and 107k miles (original owner)
her hearing is not the best for small humming droning noises but mine Definitely are lol
i noticed this past winter when driving it on the freeway the noises has gotten Much worse, especially with the windows closed

for the past few years i have noticed it sitting down at the low RPMs in D and is VERY stubborn about downshifting to 3rd gear on its own. i always have to manually throw it into 3rd to keep the momo up the hilly or it Really starts slowing down at 70mph
i have always driven MT so i am unclear about the workings of the Auto and Why this taco will not auto-shift down Unless the throttle is nearly mashed to force the downshift into 3rd



i am also open to this droning being the original muffler exhaust (perhaps the cats?) which is all becoming discolored from age and climate
as well i changed the rear and front diff fluids both within the past 2 years, that front diff was a monster to get the plugs pulled lol and resulted in injury

 

are you still on the TW boards, Cathy?
which transmission is this the 3+D speed or 4+D speed automatic

 

Sorry for the very late reply. I actually tried to type this out twice and lost it while on my phone and again while typing on my Computer. I have no idea what happed while I was typing it out on my computer, I think I hit F12 and brought up console command and wiped all my typing out.

This was more for 4.0L v6 w/5speed. This could still apply, but I am not familiar with the four-cylinder powertrains. To answer you question, yes it should be safe to do at 65mph as you only need to apply the brakes enough to cause the brake lights to come on, you are not trying to slow the truck down. But you shouldn’t have to go that fast for the clutch style torque converter slipping issues. The clutch should only go into the partial locked state while at part throttle.
Normally you will only feel it around the 45-55mph while in the highest gear, but I guess you could feel it at faster speeds if it got really bad.
Humming noise could be a lot of things, tires, bearings, driveline, transfer case, differential. This could be an issue with the engine's power, like you said, cats could be on their way out robbing your engine of power.
But if it's being stubborn about shifting, I would start with a filter and full transmission fluid change, not a flush. There are DIY's on here on how to do it, but it involves a lot of starting and stopping the engine to get all the fluid changed, it's a bit of a pain. You could take it to Toyota and have them use their machine. Just make sure they are not doing a flush. Flushes can cause a lot of issues, especially on older transmissions. To be fair, changing the fluid in a neglected transmission can cause issues too, but if you already have issues you are already between a rock and a hard spot. Just use a high quality fluid equivalent to Toyota's. I like AmsOil but there are other good product too, or you could always use Toyota's fluid. It doesn't hurt to add some extra friction modifiers too. Friction modifiers help keep the fluid from breaking down so quickly because of the clutch plate and how they pulsate.
It is possible that the fluid has been broken down long enough to cause damage to the the clutch plate and it will need to be repaired. Unfortunately modern day Transmission require more frequent serves then the older ones, even more so then the manufacturer state.
When Toyota released the Second gens, the transmission manufacturer said it had lifetime fluid, no changes needed. Now Toyota recommends changing it every 60k for normal use. But transmission shops says that's not enough, they recommend every 30k. lol

Hope this helps!