2000 Ford Focus very loud

I happily own a 2000 Ford Focus with about 76,000 miles on it. It is a manual transmission (5 speed), 4 door, 4 CYL engine. I have had no major problems with the car (besides changing the timing belt, which I’m told is a necessary thing at 70,000 or so miles, and fixing the A/C this past summer.)

My one complaint is that it gets very loud when cruising or accelerating from about 35 mph to between 45 and 50 mph. It’s not engine noise, it’s in the transmission or wheels. It’s when I get to about 2600-3000 RPM and shift up from 3rd to 4th is when it is loudest. Once I’m up into 5th and going at about 55 MPH, it goes away.

The sound is like a whirring, rotating sound - it’s hard to describe it. A friend of mine has a Subaru with a 5 speed transmission and similar mileage, and it has that same sound.

I know it’s not a problem, but I want to find out where it comes from and what causes it.

You’ve had the transmission oil level checked? Is it ok? A transmission oil change might help. Might.

It sounds like your 4th gear is worn. These helical gears are designed to be quiet, as the design allows for the teeth have constant contact between the teeth even as the gears rotate and change from one tooth mesh to the next. But, a whirring sound happens when the teeth get worn or damaged, and cannot maintain that constant contact.

Changing the oil could help reduce the noise. But, if the noise continues, the only other fix is to overhaul it.

I have the same problem with my 87 Accord. Transmission oil is fine. If my 4th gear is worn would it be extremely expensive to have this serviced ?