This one has me and all my mechanical friends (well, you know who I mean!) baffled. My 240 only does this after a few hours of highway miles. It makes a gawdawful screeching sound from somewhere in the drive train - I suspect the rear axle assy. Noise often goes away when I touch the brakes, but not always and seldom for long - the only thing that stops it completely is ending the journey. After which it doesn't do it for months of around-town, or on the way back home after a few hours of highway running. Suggestions have been dry dif (no), rock trapped twixt disk and dust cover (no) - but that certainly sounds about right. I've about given up, but it's too loud a noise to ignore - turning stereo up to max won't cover it. Any suggestions appreciated, signed hopeless.

You are the one who says it sounds like from somewhere in the drive train, therefore everyone else responds with the "usual suspects" for high pitched drive train noise.

If the pitch varies with speed, then I will chime in on the center support bearing. Most often it is described as a jet engine or siren sound.

If constant high-pitched, then I suggest it is from a wheel and it is caused by a sticky brake caliper due to rust in the ways or past the dust seal on the brake pistons.

It can be very dangerous, too, if the sticking caliper causes a front rotor and brake caliper to overheat, boiling the fluid in both circuits. You'll touch the brakes one day, and instead of the sound going away, you'll find the pedal goes to the floor.

Check for heat at the brakes with the back of your hand (or an IR thermometer) after a longish drive and keep track of calipers that wear pads unevenly.
--
Art Benstein near Baltimore

“Strong minds discuss ideas, average minds discuss events, weak minds discuss people.” -Socrates

The OP says that the noise goes away if you touch the brakes. I don't think noises from the centre bearing or the diff would be affected by a touch of the brakes. A stone scraping on the disk has been mentioned and dismissed but the240 dust shield/backplate is very prone to rust and easily end up touching the disc. Many owners just trim them off and say the cooling is better. With a fraction of extra clearance in the wheel bearings, the disc can centralise for a while and take it away from the rusty shield. Easy to check and if you can get the front wheels on the brake test rollers at the garage you can maybe localise the sound. Worn down pads going metal to metal mentioned and if that's the case shame on you for letting them get to that point, but again, an easy fix.

It is all in the description of the sound. Converting a sound one has not identified to language. The manufacturer attempted to build this translation capability:



Many of these sound issues clear up when the OP makes a recording and uploads to youtube...
--
Art Benstein near Baltimore

“I cannot teach anybody anything. I can only make them think” -Socrates

Love that diagram!

The one time I experienced a center support bearing failure it was a 15, 14 and then a loud 9 (buzz, squeal, BOOM) all in the span of about 30 seconds IIRC. Those noises were clearly coming from right between the front seats, not from the rear like the OP reports.

Troubleshooting of sounds from the other side of the interwebs is indeed a challenge.

Regards,
Will

In order of probaility.
1)Center Support Bearing
2)U- Joints, esp if you have never lubed or replaced them
8)Very Worn down rear brake pads
25) the rear axle assy.

Search "driveshaft support bearing" here.

I drop the center support, pry the cap off, clean and regrease.
--
240 drivers / parts cars - JH, Ohio

Hi,

Yes it's like the other poster said, its the support roller bearing in the middle of the driveshaft.

It has a roller element inside that is dry of lubrication or has dirt in it.
One or more elements is sliding and not rolling! As they develop more flats or those that get more flat it will happen more often until it doesn't go away.
By then, it's liability to keep turning drops significantly, until it rips the whole rubber support out and the driveshaft starts swinging about violently!
It will literally beat the **** out of the bottom of the car and can even go as far to bend/break the driveshaft!

Right now it's only screaming "Take a Look at ME!"
You don't want to see any other type of protest demonstrations!
Sometimes people just try to squirt oil in there but that just makes for a slippery slope in more ways that you can count!

Fate has its ways of giving out "fair" warnings, all the time.
Being wise, is to act upon them! Wisdom is gained with age or we don't do that anymore, gracefully!

Over my 40 years, I have seen a couple Volvos, not mine, on the side of the highway with lumps underneath!
It's not graceful, no matter what kind of vehicle!

Phil

Driveshaft support bearing can make this noise.