Is this serious?

I have a 97 ford explorer sport that recently had the fuel pump replaced. 4 days later i was driving down the freeway when it began making a loud reving noise and then stopped. I listened for it again, and the next time it began, i pulled over and checked my transmission fluid levels. All good and the color was pink. the noise seems to be coming from underneath the drivers side. the rpms dont jump up when the sound comes on, and the truck doesnt lose any power. a friend rode with me and said that it may be the 4wd engaging automatically. as this is a AWD truck there is no shut off switch. Any Ideas on what this could be?

This could be serious. Since it is AWD, this truck has a center differential and a front and rear differential. I know the rear differential has it’s own case and gear lube in it. I’m not sure about the center differential and front differential. These will need to be checked. I’d take it to your mechanic and explain it to him.

Fuel supply

Wait. Its not a two stroke…ok never mind…?

the fuel pump was just replaced, and all fluids (exept for differential) have been changed out with in the last 3000 miles…thanks for the help. Keep the ideas coming.

Is there a correlation to vehicle speed or engine RPM or temperature? Have you looked under the vehicle for loose parts or suspicious markings?

Everything was normal except for the noise. the rpm wasn’t going through the roof, and the temp of the truck didn’t seem abnormal. There was nothing odd when i took a look under the car either (Not that I would know what to look for).

For peace of mind take the truck to a trusted mechanic.

The noise could be as simple as a cooling fan going bad. But get it checked out ASAP.