So I've been hearing a inconsistant humming sound from the what sounds like the rear end... the thing is it may be my tires because I just found out that although they have lots of thread life they are all cupping. It seems like they were bad from the beginning.... they cause a vibration through the vehicle that gets stronger at highway speeds.... the humming sound I haven't heard for a long time but when I rotated the tires 2 front to back the sound returned but its not consistant. My first thought is the tires could be causing this sound as well (these are directional tires) If it's something else I won't know for sure until I get rid of these tires.....my rear brakes were done before I got the car so I'm not sure what type of brakes were used...they do squeal a little so they may not be OE which I understand can cause this humming sound as well... any one had this problem?.... but here's what I found when researching the issue... this may be of help to someone else as well


The loud resonant hum you are experiencing, especially if it is temperature sensitive, is a known problem with the model vehicle you have. The cause is the brake pads in the rear of the car. The only known way to resolve this issue is to install the OE volvo brake pads and shim kit available from the dealer; it is critical that the special silicone grease be installed on the pad and shim backings as well, to allow the various components to move freely to help absorb vibration.

Rotor construction is critical also; if you have installed discount Chinese rotors they may not be the same mass and thickness as the OE, which contributes to vibration related issues.

Apparently on this vehicle, the design allows a vibration to build that when it reaches a specific frequency then starts to resonate throuth the springs and vehicle body. Such a resonance issue may be temperature dependent due to clearance and size changes in various components that are involved caused by temperature variations.

Here are a couple excerpts from a repair database where professional technicians exchange repair information, regarding this problem on 97 Volvo 850 models:

"In each case the fix was the correct compound
factory rear pad. Not sure if it applies to your case, but we have found
that
850s are sensitive to rear pad compound"

"Did you install the brake shims that are available? Did you use Volvo pads?
It sounds like the problem is a vibration from the pads."

"We used to use the 240 rear brake pad in all 700 series and 850s
(they are the same size, and shape, different compounds) but we quit using
them on 850s after we had a few that would make a weird resonant hum from
the
rear when nearing a stop. In each case the fix was the correct compound
factory rear pad. Not sure if it applies to your case, but we have found
that
850s are sensitive to rear pad compound."

"Replace the rear rotors and pads
with Volvo parts.
Use the rear pad shim kit p/n 272272.
The Note for the shim kit says,
"Eliminates rear axle vibrations when braking"
You could try the shim kit first, then replace the
pads and rotors if brakes still vibrate."

"The vibration is caused by warped rotors, even after
turning it is still there. The vibrations are
frequency resonance that are just at the right pitch that
sets off the coil springs vibrating. It's like hitting a
a big tuning fork that just keeps getting worse
the more that you stop.

The shims seem to absorb or change the frequency just enough
to offset the tuning fork effect.
Replacing pads, rotors and using the shim kit will correct the problem.
Look at the shim and the arrow goes up.
Volvo does have silicone brake paste that we put on the
backside of all new pads and shims in the contact area
of the caliper pucks and brackets."

 

You likely have a worn out suspension mount that is transmitting noise through the car. If you can, post a video of it so we can hear it.

 

Never thought of that... so are there suspension mounts on each wheel? I don't think I had these checked. Also in the last couple days my Brake light has come on ...each time I topped off the fluid because it was just above the Low mark but low enough to trigger the sensor at times... the mechanic said he saw no leaks and there aren't any leaks in my drive way... So where is this fluid going? The brake feel hasn't changed and the brake pads have a ways to go front and back. What are your suggests? Power steering fluid also needs to be topped of maybe once a month but hey the car is 15 years old ...I'll have to keep monitor that to see if it becomes more frequent ... I wonder if there are any stop leak fluids for this type stuff

 

They shouldn't be intermittent but there is also a chance you have a bad wheel bearing.
If you jack it up and spin them you might hear something.

 

The only time your brake fluid should get low enough to trigger a low fluid light is when all 4 pads are low. You need to find out where the fluid is going. I don't know of any brake system stop leak. That situation can be fatal.

As for the power steering, they have a lot of stop leak products for that. I have to add fluid to mine every 2 weeks.

The real end has lots of mounts back there, including shock mounts.

 

If you are adding brake fluid, especially every day or ever other there is a leak. If no one can see it than the likely place is into the brake booster. The seal at the back of the master cylinder starts to leak and the vacuum in the brake booster the master cylinder is mounted to sucks it inside.

Brake system stop leak. It doesn't exist. It's like lazy people who put on compression fittings instead of properly flaring and using a flare fitting. Compression fittings are good for household plumbing but can't handle the pressure from an anti-lock brake system when it's pulsing. It's a death sentence looking for a place to happen.

Power steering stop leak, lots of products you can try.

 

I was right! When the brakes were done on the car new brakes lines were installed and it turned out that one of them just needed tightening ... problem solved with no money spent thank God!