I have the same problem on my 2.0L 2001 Legacy auto. It began about 1000 miles ago. It has now done 84k. From my online searches this is most probably 'piston slap', and is not serious. To save weight Subaru shaved down the skirts on the pistons. With normal engine wear the pistons will at some point rattle against the cylinders until the engine is heated up and the pistons expand. The rattling should go away after a few minutes. If it persists say 10-15 minutes, it may be another problem.
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Re: Engine make Knocking noise when cold
I could be one of 3 things, #1 lifter might loose prime when it sets for a while like overnight, #2 could be rocker a little loose witch is part of lifter that runs on a pushrod, and #3 could be a ristpin witch holds piston to connecting rod a little worn. None of thies are a real big thing unless they get louder or longer in duration. I don't know what kind of miles are on it but what you describe is usely associated with high millage. Just keep your ears open.
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The 3.4L engine has special pistons that are designed to expand as the engine heats up. When the engine is cold, the pistons make a knocking sound that resembles lifter noise or piston slap. As long as the noise goes away after the engine warms up, don't sweat it; this is normal.
On a very cold start (eg winter) older Subs often make knocking noise from the LHS of the engine. This is actually quite normal and is a "feature" of the oil paths in the engine. It goes away once the engine is warm.
Might also be the cam belt hydraulic sensor working loose. The tensioner pivot bolt can be done too light or fitted clumsily, and it strips the thread. If it gets too loose, the belt can jump teeth and bend valves.
Ever had your wedding band on during the winter outside and it wants to fall off ,same thing for cars metal expands and could cause some unwanted noise from heat shields and such thing is to find it and eliminate this sound 1 morning you must find this sound and where it is so you do not confuse that sound with an actual rattle inside the block so please find and fix this problem you will thank me later. Thanks in advance please remember to rate and leave testimonial this is
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might be the valves have been damaged running without oil
- or the crankshaft / camshaft / main bearings
most probably you have to let the engine be rebuilt by a mechanic
(expect cost of more than US$ 2.000)
or get another (used) engine at a scrapyard
The lower end of your motor that has the knocking noise will sound like a deep penatrating thud. That will be the rear bearing for the crack shaft. You can find a cheaper place to have it repaired. $3,500 sounds more right to repair at most places
knocks when cold ( Start ), then sort of diminish or disappears when warming up ???
Knocking goes faster when accelerating when cold ???
If Yes, then Piston knocking on the wall of the cylinder . The sound disappears as the engine is getting warmer it
dilates and the gap gets smaller.
If this just started,
1. Put quality oil & filter ( Mobile Full Synthetic Oil & Mobile Oil Filter or Purolator )
2. Do not put cleaning agents mixed with oil.
3. Personaly i've put "Restore" Engine Restorer & Lubricant in cars with more then 200 000 km and the results are amazing. But only "Restore". My Volvo is at close to 300 000 km, and it runs great. It's not a cleaning agent.
By experience if the knocking just started you can put an easy extra 100 000 km on the engine before a rebuilt. Quality Oil & let the engine warm up ( 3 mins ).