I hear noise from the rear when I drive at 120km/hr. The noise lasts up to
150 km/hr and than disappears.
It is kind of like differential or bearing grinding noise.
What could be the reason. Maybe not enough fluid in the differential, or
bearings. Anybody else had this problem?
The car is automatic.
It's Probably (but only probably) not a wheel bearing. The sound from those
will likely be noticeable at lower speeds as well. Possibly a differential
going out. Unfortunately, with differentials, when it gets to the point
where you hear the noise, it's too late to just top off the fluid; you're
running on borrowed time. I'm not familiar with the limited-slip setups the
Volvo's sometimes use. If your car is equipped with one of these, something
in there might be causing the noise.
But there are a few other possible causes for the noise. Driveshaft
U-joints or the carrier bearing are possible candidates. If the driveshaft
itself has been dinged, it might be slightly out of balance, which could
also give the symptoms you're describing. A tire going bad might even do
it. Those last two would probably be accompanied by some noticeable
vibrations, however. I've even seen a loose trim piece vibrating in the
wind mimic a grinding mechanical component at high speeds. Also, grab your
tailpipe (make sure it's cool, or wear thick gloves) and shake it vigorously
to see if there's anything loose and rattling in the exhaust system.
Sorry to make things more complicated instead of easier. It's all wildcard
guesses without being able to hear the noise. A good idea might be to find
a trusty mechanic and take him for a ride.