My Ranger has 212k. Bought it in June with 209k. 98 2.5 5 speed.

A couple months ago I changed the trans fluid with Walmart Dexron, which was supposed to be compatible. The old oil was black, and it shifts really nicely now.

However, on cold mornings, there is a whirring sound whenever you let the clutch out in neutral, or while moving in any gear except 4th, even coasting in neutral. Sounds like static on the radio.

I'm not sure if it made the noise before the fluid change. It wasn't cold then either. It goes away after you drive awhile.

Is this normal for a trans with high mileage? Wrong fluid maybe?

My last truck always made a similar noise, but it was a completely different animal, GM NV4500.

 

Reads like counter shaft is getting a bearing noise, also could be input gear, it spins the counter shaft

On a 5-speed all the gears use the counter shaft except 4th, 4th is 1:1 ratio so input shaft(engine) and output shaft(drive line) are connected directly together so there is no stress on the counter shaft.

I am sure changing the fluid wouldn't cause the problem but may allow you to hear it now.
Mercon ATF was Ford spec, some say doesn't matter other says it has to be Mercon ATF, all are anecdotal stories so...........

It can run fine for years so I wouldn't sweat it for now
If it gets louder then you will need to deal with it by pulling out the transmission, and either rebuild or replace.
The M5OD-R1 from a 1989 to 2000 2.3l or 2.5l will fit your '98 2.5l, 2001 and up 2.3l transmission won't fit your engine bolt pattern.
M5OD-R1 from a 3.0l or 4.0l won't fit either
Also needs to be from a 2-wheel drive model

M5OD-R1 is a very good transmission contrary to what you may be experiencing now, lol, so used models are a fairly safe bet but it is still a bet/gamble since you have no history.

Just for FYI, Google: how manual transmissions work
Some good and short animations that will give you an idea of what could be making the noise, when and why
M5OD is a constant mesh transmission

 

Forgot to mention but the noise also disappears in second. Most noticeable in 3rd and 5th.

I'm not too worried about it right now. All I can hope is that it lasts a couple more years, at which point I'm sure it'll be due for a clutch as well.

 

Should be fine, M5OD's are good transmissions, but everything that runs that hard will wear out