I have a 98 ford explorer sport, Im hearing a reving noise from under the truck when I'm around 30- 40 miles per hour. It sounds like its missing a gear but I'm not sure is this the trans?

I have a 98 ford explorer sport, Im hearing a reving noise from under the truck when im around 30- 40 miles per hour. It sounds like its missing a gear but im not sure is this the trans?
Is your truck equipped with a tachometer? Does the engine rpm flare when you notice these other symptoms?
The problem may be something slipping in the trans but could also be a worn/failing fan clutch. Do you know how to test the fan clutch? It's quite easy, but you're certain the problem isn't that, we won't bother.
Let us know.

Is your truck equipped with a tachometer? Does the engine rpm flare when you notice these other symtoms?
The problem may be something slipping in the trans but could also be a worn/failing fan clutch. Do you know how to test the fan clutch? It's quite easy, but you're certain the problem isn't that, we won't bother.
Let us know.
How do you check the fan?

I have a 98 ford explorer sport, Im hearing a reving noise from under the truck when im around 30- 40 miles per hour. It sounds like its missing a gear but im not sure is this the trans?
How do I check the clutch fan?

how do you check the fan?
Safety is paramount.
With the key and the engine off, while the vehicle is still at a normal temp (not extremely hot nor extremely cold) spin the fan blade by hand. A light resistance should be felt. If there is no resistance or very high resistance, the minimum and maximum fan speeds must be checked.
Checking fan speed while operating requires special tools and experience, but if it passes the manual resistance test but is still suspect, you can try the following: Run the engine till it is at operating temperature. Park the truck and open the hood so you can hear the change better. Run the engine up to about 2500 RPM and hold that speed. Within a moment or two, you should be able to detect a change in sound coming from the fan. If you don't find any speed change, try around 2700-2800 RPM and listen again.
Let us know if this does anything.