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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8569
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi Montero: 2003 Montero. Rattling sound from engine

Customer Question

2003 Montero. Rattling sound from engine @ cold start. Will quiiet down after 2-3 miniutes. Hard to tell origin of noise. Doesn't do it every time.
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Doug replied 1 year ago.
When the rattling is occurring is it always completely rhythmic (no random stopping/speeding up/etc by itself without doing anything?
Does the rattling keep pace with the engine... if you rev the engine does it increase in speed?
Customer: replied 1 year ago.
Doug,The rattling is rhythmic, and does vary with the speed of the engine. It is loudest at start-up and remains that way for a couple of minutes. It gradually fades as the engine begins to warm-up and the idle speed decreases. What is puzzling, is that it is not consistent, at least to this point. Sometimes the engine starts normally without rattling, sometimes not. Initially, the engine would make this rattling noise for perhaps thirty seconds, then dissipate. I first noticed it upon start-up in the winter, especially when the temperature is below zero (okay, I live in North Dakota). Recently, the noise occurs more frequently and persists longer, even as the temperatures have increased. The engine (3.8) has about 110K and uses about a quart of oil every 1,000 miles.
Expert:  Doug replied 1 year ago.
The oil consumption is about typical on this model... even when new/under warranty the "Acceptable" rate of consumption was 1 quart per 1000 miles. I would want to check carefully for leaks though, as you are on the upper end of the spectrum there for consumption. While these engines are not shy about burning oil, leaking is a bigger problem at this mileage normally.
It sure sounds like you are probably hearing lash adjuster rattle. This occurs on start up and relates to engine speed, then slowly fades as the engine warms up. If you restart the engine warm it usually is quiet then, as well in some cases when parked at unusual angles it will be quiet on cold starts too (due to the oil not draining back out of the lash adjusters).
What I would do at this point would be to get it to a shop that can listen with a stethoscope on the valve covers and confirm the noise is coming from there. Knowing it is engine-speed related and that it comes and goes primarily with temperature fluctuations, that is really about the only consideration that is common (a belt/accessory issue wouldn't be temperature influenced, etc).
You can sometimes disguise the noise with slightly thicker oil and bleeding it, however gien your climate this is not going to be an option for you (0F temperatures and 20w50 etc do not mix).
Once confirmed, replacement of the adjusters is the way to go (24 total requires pulling valve covers off). Just be aware that some noise from them is normal and it will never be completely silent... but in your case it definitely sounds like it has progressed beyond what we would consider even remotely typical.