Top strut mounts and upper shock mounts: when to replace?

In a car, a strut is a shock absorber that is built into an assembly with the coil spring and the top mount, called a strut mount, see the illustration. A top strut mount connects the strut to the vehicle's body or frame. 

In many cars, you can see the front top strut mounts under the hood (see photo below). Shock absorber mounts serve the same purpose, scroll down to read more about them.

Strut Mounts
Strut mounts have multiple roles: First, they have to keep struts firmly in place and absorb some of the shocks from road bumps.
Second, in some cars front strut mounts have strut mount bearings to allow rotation together with front wheel when turning left or right.
Third, strut mounts have to provide good sound insulation, or the road noises and vibrations will travel from the struts through the vehicle's body and into the cabin. For this reason, strut mounts have a rubber cushion, so there is no metal to metal contact between the strut and the body.

Strut mounts don't need any maintenance. They don't need to be replaced unless they are worn out or there is some other problem with them.

There are several reasons why strut mounts may need to be replaced. The most common is when a worn out strut mount produces a clunking or rattling noise. 

A bad strut mount bearing can also cause a popping or creaking noise when turning the steering wheel at slow speeds. The way the steering wheel turns may feel jerky too; like it jams and snaps free repeatedly. This happens when the grease inside the bearing dries out or when there is corrosion or wear inside the bearing. 

Again, this type of noise might be difficult to diagnose, because many problems can cause a similar noise.
In some cars, a strut bearing comes with a strut mount as one unit; in other cars, it can be replaced separately, although the labor is the same.

Strut mounts are also replaced if they show excessive corrosion or if the visible rubber part looks ripped or badly worn. Rear strut mounts, for example, are often replaced because of rust damage.

How long do strut mounts last? In some cars, strut mounts can last for the lifetime of a vehicle, while in others they are known to fail sooner. It also depends on the quality of the roads. Potholes and speed bumps can cause many parts of the suspension, including strut mounts, to wear faster.

Upper shock mounts
In some cars, the top ends of the rear shock absorbers are connected to the car body through upper shock mounts. 

You can find this type of shock mounts in many vehicles, including Mazda, Ford and Volvo. They serve the same purpose as the strut mounts: to provide a connection point between the shock absorber and the car body without metal-to-metal contact.

In older cars, it's not uncommon to see upper shock absorbers mounts breaking off due to excessive corrosion like in this photo. This causes a loud knocking noise in the back of the car when driving. Obviously, it's not safe to drive like this. In some vehicles, the upper shock mount comes as a separate part; in others, it's sold as one unit with the shock absorber. Replacing a rear shock absorber in an average car will cost $140-220 for one side. Replacing only the upper shock mount will be slightly cheaper.