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1981 Subaru GL knocking/loud clicking noise

I hear a knocking noise while driving. The noise occurs only when accelerating. The car idles, shifts and drives great but has the knocking noise. Once my foot is of the gas pedal, the noise goes away. I checked the heat shield, it has clearance. I jacked the rear end of the car up and rotated the wheels, no noise.

I have replaced the front drive axles(wheel bearings, cv joints)new plugs, wires, cap, etc. She runs great but the knocking noise.

Any suggestions(only nice ones)?

Thank you,

Subaru SOS

BTW- It is a 4 speed, 4 cylinder, with optional 4wd.

I would be looking for a bad CV joint. Any difference when turing right vs left?

You might clarify a few things.
Is this a click or a knock?
Is there any noise at all when the vehicle is sitting at a stop?
During the halfshaft/CV joint replacement did you make absolutely sure that the beveled holes on the halfshaft were mated with the beveled holes on the stub axles which protrude from the transaxle? (If done properly, the serrated DOJ pins should tap back into place pretty easily.)

How many miles on it??

Motor mounts or transmission mounts been checked?

Whenever the gas pedal is applied(going straight or turning).

Louder than clicking but not a hard knocking noise. Only makes the noise when the gas is applied. Sounds great sitting still. The pins went in very smoothly.


I quickly checked the transmission mounts(not very thorough check). I have not checked the motor mounts.

Subarus can be prone to center main bearing knock, even on engines with much fewer miles on them. This often shows up as a muffled knocking sound.

To determine if the center main bearing is the cause of this noise disconnect the Number 3 spark plug wire and start the engine up.
It will run rough of course but if the noise goes away the center main is the cause. They can go forever like this so it’s not necessarily a reason for concern.

If the noise remains the same there could be an issue with the valve train; either the valve lash or the sideplay in the rocker arms where they’re mounted on the rocker shaft.
Loose valve lash should be very noticeable at idle speeds though when the car is not moving.

Ok, remove the #3 wire and drive or idle?

Thanks for your knowledge & time on the subject.

You do not have to drive it. Just rev the engine a few times and see if the noise goes away.

The reason for the No. 3 wire is because that particular cylinder exerts the most pressure on the center main bearing due to the engine design. When that cylinder is not firing much of that pressure is removed and the noise goes away or subsides a lot. Hope that helps.

It only makes the noise when driving(uphill,downhill,flat roads, all the time). Sorry, I know I said while giving it gas. I should have been more specific.

Maybe what we’re dealing with is a pre-ignition rattle. This is generally caused by a faulty EGR system or the ignition timing being too far advanced.

Since this model is distributor equipped have you or anyone else messed with the distributor in any way? Disconnecting the vacuum lines to the distributor vacuum advance is very critical on these cars when checking and setting the timing. If the timing is set with the hoses connected the timing will have about 10 degrees too much advance and eventually this could destroy an engine.

An EGR fault could do the same thing. To test the EGR connect a piece of vacuum hose to the EGR and suck on the end of the hose while the car is idling. The engine should stumble and die, or attempt to die. If nothing changes then the EGR is at fault.