Hello I have a 1993 f-250 with a 7.3 nonturbo and a 5 speed manual. When traveling 65-70 and around 2300 rpms on the interstate I have a high pitch wistling sound not sure where it's coming from. What I do know is it only happens when you are cruzing at that speed. If I push down on the clutch and hold the rpms at the same as they are while driving it goes away. I I start accelerating it goes alway and if I take my foot of the pedal it goes away. Drives good shifts good. What could could this be? Thanks

 

Was gonna say door or window seal... I assume you're in 5th gear. Does the noise go away or change if you maintain same speed in 4th gear? How about same RPM in 4th gear?

4th gear is direct straight through the trans with no load on the countershaft or gears turning at different RPM on the mainshaft.

 

I cant go 65-70 in 4th gear. Haven't tried to do the same rpms in 4th I don't think so just 5th. But if I am going 65-70 and push the clutch in and let it coast at 65-70 the noise goes away.

 

With 4.10:1 gears 4th at 3300 RPM should be about 74 MPH, 4th at 2300 should be about 52 MPH. Unless you've got lower gears it should be able to do 65 MPH in 4th gear. More important to diagnose transmission noises is RPM and load. Try to reproduce the RPM and load while in 4th gear. Also do the same in 4rd gear.

If the noise is present in 3rd and 5th but not 4th you have a countershaft problem. Once done diagnosing, drain the trans fluid and inspect for metal. You can also pull a PTO cover and wiped the bottom inside of the trans with a rag and look for shiny metal. Determine if it's steel or aluminum to get a better idea what type of failure you may have.