2003 Ford Explorer Grinding when accelerating

Hi all. My 2003 Ford Explorer is experiencing some weird stuff lately.
Recently, I took a trip out of town on a road with high speeds and many hills  
on the way; on one of the hills, traveling at about 45 mph and at about
3000 rpm I noticed a weird grinding sound coming from my engine when it
slipped into a lower gear to make it up the hill. I would let off the gas and it
would be fine, but once I tried to accelerate again it would do the same
thing.
It shifts fine and does not do this at higher gears and speeds, just only
when I accelerate and really try and make the Explorer really go. I noticed
too after this happened, on the same trip, that once I started from a dead
stop on flat roads it did the same thing when trying to accelerate but then
stopped again once reaching the higher speeds. I also discovered that
when I have it idling in park or neutral and press on the gas, the same
noise still can occur at around 3000 rpm. But when i turn the car off and
just drive it again the next day, this does not occur. So it seems pretty
random to me.

Anyone have an idea? The previous owner took very good care of it, didn't
drive it too hard so I know it's been well maintained, but he mentioned his
mechanic said the torque converter might become an issue. Could that be
it? I figured it probably isn't the transmission because it shifts fine and
doesn't hesitate like what I've experienced with a previous transmission
problems.

Any help/ideas are greatly appreciated!!