Subaru Impreza AntiRoll Bar Bushes

Subaru Impreza Suspension Knocking? 5 Possible Causes and How to Diagnose

A knocking noise whilst driving your Subaru, especially at low speeds is unfortunately fairly common for the Impreza models and can be extremely annoying and frustrating to find the cause.

This article lists 5 common causes of suspension knocking noises on the Subaru Impreza Turbo, WRX and STI models and how to find each one. We also some product links for replacement parts.

Before we start we should add that this work is best carried out by a specialist workshop or Subaru dealer, as its much easier to find the cause with a vehicle ramp and some replacement parts require specialist tools, such as a Suspension Spring Compressor, however this guide should be worth a read as it may help you with identifying the cause. Also, if you suspect a worn component we strongly advise having your suspicions confirmed by a qualified mechanic before spending out on new parts.

1. Check What’s In Your Boot

impreza-boot Subaru Impreza Suspension Knocking?  5 Possible Causes and How to Diagnose
Check whats in your boot – anything loose and rattling?

This may sound so obvious as to not be worth checking, but before you do anything else, if the knocking noise is coming from the rear open up your boot, take out any items in the boot, lift the spare wheel cover and made sure items such as the tool kit, jack and spare wheel are all secure. In fact we suggest completely emptying the boot, including spare wheel and testing again to see if the noise is still there.

At the worst, it’ll prove not to be the cause of the problem and you’ll end up spending a few minutes putting the gear back in your boot but its definitely worth the time to check. Several years back a customer drove from an hour away for us to diagnose knocking, suspecting rear shocks. The issue was caused by his loose wheel jack rattling against the mount.

2. Droplinks

droplink-subaru-impreza Subaru Impreza Suspension Knocking?  5 Possible Causes and How to Diagnose
Subaru Impreza Droplink

Worn or loose droplinks are one of the main causes of suspension knocking noises, and if left the noise will generally get louder and more frequent over time. Thankfully they are fairly easy to check and as long as not seized in place can be changed in about 30 minutes per corner.

A droplink is basically a rod with some form of pivot point at either end which connects your suspension arms to the anti-roll bar. The droplink usually has a bush with either a captive bolt or a standard nut and bot setup to connect to the anti-roll bar at one end and suspension arm at other. Worn bushes in these end can cause excess movement in the link and hence the noise. Other causes can be droplinks that are hitting or touching the anti-roll bar or droplinks that have a broken rod between the two ends.

The best way to check these is to get your Impreza to your local workshop garage and up in the air on their. If you don’t have this option you can check them whilst on the floor (assuming its safe to do so) but as the Impreza sits low there’s not much room to see. Work out where the sound is coming from as best you can then take hold of the droplink and try and move it to establish if there’s excess movement. This is so much more easily done on a vehicle ramp.

If excessively worn its likely the cause of the noise, then you can choose either a genuine Subaru replacement or an aftermarket equivalent, with prices starting around £35.00 per link upwards for a pattern part. Or you can replace both sides with uprated versions for usually not much more money, such as those available from Whiteline at this link

3. Anti-Roll Bar Bushes and Mounts

anti-roll-bar-bushes-subaru-impreza Subaru Impreza Suspension Knocking?  5 Possible Causes and How to Diagnose
Subaru Impreza Anti-Roll Bar Bush and Mount

The Anti-Roll bars, front and rear are secured to your Impreza’s chassis by means of a rubber D-shaped bush in a clamp. When the rubber bush becomes worn the anti-roll bar can move in the mount and cause a knocking noise, which will get louder and more frequent the more worn the bush becomes.

The rear bushes are easy to inspect by looking underneath the car with a torch, and attempting to move the anti-roll bar by hand whilst observing any play in the bush.

The front bushes are more difficult due to the location, so we’d recommend getting the Impreza up on a vehicle ramp to carry out a proper inspection.

Replacing either front or rear is a straight forward job (assuming no bolts are seized) and replacement bushes are available from either your Subaru dealer and our own website from leading suspension tuning brands such as Whiteline, Powerflex and SuperPro

Bushes come in either original rubber form, essentially the same as was fitted when the car was built or uprated polyurethane which is a much stiffer material. Benefits of using the polyurethane bushes are that there is less distortion of the bush therefore less movement therefore making a slightly firmer ride.

4. Shock Absorbers

shock-absorber Subaru Impreza Suspension Knocking?  5 Possible Causes and How to Diagnose
Subaru Impreza Shock Absorber – A common cause of knocking

Worn front or rear Shock Absorbers are a common cause of suspension knocking, especially at low speed and it seems more common on the 2001 onwards models. After checking the above 3 possible causes, its time to consider whether the shocks are causing your knocking noise. Besides the noise, some of the other symptoms of knocking shock absorbers are a suspension corner that doesn’t return as smoothly as the opposite side, or appears to stick briefly after load is applied before returning to its original position. You can try bouncing each corner by pushing down firmly on an area that’s structurally sound (i.e. not a wing or bonnet or panel that’s likely to dent or flex) and checking the shock returns smoothly, without knocking, and that it doesn’t stick on its return. However the Subaru Impreza shocks are quite stiff to its difficult to carry this out accurately.

Another way to tell is to look underneath the car with a torch for any damp patches on the suspension and also check the shock piston, to ensure there are no signs of sticking upon it such as ring marks.

Although there are guides on adding a grease nipple to shock absorbers and filling the unit full of grease we’ve decided not to cover it here, but you might be able to find it through a search. We’ve no experience of this method of restoring shock absorbers or extending the life of them.

Shock Absorbers can prove quite expensive to replace, with price starting at around £96 upwards per side for the part, for example see here however they should be replaced in pairs, so both rears done together, or both fronts. Costs of labour are around 1.5 hrs per shock absorber to remove an old unit, compress spring, remove top mount and re-assemble new, though this might differ from workshop to workshop.

To replace the shock absorbers you need access to specialist tools such as coil spring compressor, therefore its not a job for the home DIY mechanic, unless you’ve experience of these and a comprehensive set of tools.

Once replaced you’ll also need your wheel alignment at least checking and likely re-adjusting, so you’ll need to factor in costs for doing so, which usually range from around £75 for a basic alignment up to £150 for a more advanced service.

If you’re unsure take your Subaru Impreza to your local Subaru Specialist or Dealer who will be able to confirm the cause of the issue, though you’ll likely be charged for their inspection time.

If you’re in need or replacing shock absorbers, you might want to consider whether to replace all 4 with an adjustable coilover system system such as is offered by BC Racing, Pedders or Tein. See our article on Suspension Upgrades for more information on this.

5. Top Mounts

topmount Subaru Impreza Suspension Knocking?  5 Possible Causes and How to Diagnose
Subaru Impreza Front Top Mount

The Subaru Impreza Top Mount, as the name suggests sits at the top of the suspension strut, compressing the spring onto the shock absorber (for those cars with a spring-over-absorber setup), holds the shock to the chassis of the car via 3 studs, and contains several key components including a large bearing which if worn can be the cause of the problem.

When worn these can produce a knocking or loud clicking sound and you can usually hear the top mount making the noise simply by listening closely to it as a friend either turns the wheel lock to lock (for front issues) or bounces or puts some load on the suspension (for rear).

If worn you’ll need to contact your Subaru dealer of local parts supplier for replacements. Labour time is again around 1.5 hrs per top mount to replace.

Did this guide help? Did we miss anything? Please let us know in the comments section below

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