My 02 Ford Focus makes a racket in 5th gear. The engine sounds like it’s ready for takeoff. The other gears are quiet but when I pick up speed, the noise is a steady & relentless hum.
Check the oil level in the transmission.
How many miles are on this transmission? If the rest of the car warrants it, a rebuilt or used transmission might be in order. If you are trying to eke out a few more years of use, you might just use 4th until that starts making noise. Fuel mileage will suffer but you might be able to make the car last a few more years.
If 4th gear is quiet at the same road speed, it is unlikely to be a problem with the bearings or final drive gears. That leaves a worn 5th gear set i.e. input and output gear. It appears that these gears can be replaced separately from the shafts and other gear sets. Although the transmission and the input and output shafts will have to be disassembled just to replace these.
Manual transmission, right? The thing about 5th gear, at least in many cars, in that gear is configured so the transmission output shaft is more or less directly connected to its input shaft, which is the same as coming directly from the engine. The engine crankshaft and transmission output shaft both turn at exactly the same rpm. In other gears the power goes to an intermediate shaft, then to the output shaft. So it’s conceivable a transmission with something failing in the input/output shaft could be less noisy in other than 5th gear, lower gears where the intermediate shaft is used to take some of the load.
As posted above, first thing to do is check the transmission fluid level. If that’s ok, on an 02, might be worth a chance to replace the fluid. While doing that the shop could use a magnet to look for signs of a failing transmission, like metal shavings in the drained out fluid.
There’s a chance this is an engine problem too, but start with the above ideas probably.
The thing about 5th gear, at least in many cars, in that gear is configured so the transmission output shaft is more or less directly connected to its input shaft, which is the same as coming directly from the engine. The engine crankshaft and transmission output shaft both turn at exactly the same rpm.
I would challenge you to find any FWD transverse manual transmission with this configuration.
@GeorgeSanJose, 5th gear is an overdrive gear and is not direct drive. So there is a gear in there. First is to check the gear lube level. If proper level doesn’t quiet it down, internal repair is required due to gear damage.
I looked through my Chilton’s manual for my Corolla, which is a 5 speed manual transverse fwd. It said in the introduction to the manual transmission section that it used a “counter-shaft” in all but 5th gear. I presume this is the same term as an “intermediate shaft”.
But later, where it explains how to take the transmission apart to replace damaged gears, synchronizers, and bearings, there’s no mention of any countershaft or intermediate shaft. The diagrams show only an input shaft and an output shaft, next to each other, in parallel. I presume this is what @insightful means, on that configuration space is tight so it is better to eliminate countershafts and intermediate shafts and just route the power for all forward gears via different configurations of the input and output shafts.
In any event, at least on the Corolla, the 5th gear routing uses parts not used in the other gears, so a 5th-gear-only noise pretty much would to be coming from one or more of those particular parts. The transmission power routing varies car to car, and I don’t have access to the diagrams for the OP’s Ford, so can’t speak to which those parts are for that model.
If 5th gear is Overdrive it is by definition not direct drive.
Often, wear on the input bearing and input/output pilot bearing are most noticeable in Overdrive because there is much less engine noise when cruising and while often unnoticed the noise in reverse is the loudest. There is a great deal of consensus here that the oil level in the transmission should be checked and for now @zengirl is remaining somewhat shy but if/when it is checked topping it off could possibly cure the complaint or, if the level is significantly low and topping off doesn’t improve the situation it will indicate that she can look forward to replacing the transmission in the near future. And if that is the case we will again lament a motorist’s suffering as the result of poor maintenance.