Where does engine noise come from?

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May 4, 2005
This has been a point of real curiosity for me. New engines run almost silently. As they age they get noisier, even if they continue to run as perfectly as ever. As an amateur home mechanic I take pleasure in a nice-running vehicle with minimal wear, and I see increasing noise as an indicator of wear, which I'd prefer to address in any way reasonable. Question is, what is the source of the increased noise as an engine wears? Some contributors: - Pulley and accessory bearings. - Belt-on-pulley noise. - Internal engine noise. - Exhaust noise. Any of these might be lessened or worsened by worn noise-damping components like rubber mounts, etc. So, for each of these sources of noise, I am curious: How much of the overall noisiness does it contribute? What is the best way to address it, and how much difference will it make? Are there any noise-related tips and tricks that might be relevant? On my own cars I try to get things running nice and quiet by making sure engine tune is perfect, using an oil that helps it run smooth and quiet, and using MMO, which has the benefit of quieting the valvetrain. I also make sure the exhaust is in good shape and replace any obvious excessively noisy external components. Nevertheless all my cars, with mileages ranging from 130k to 240k, are much noisier than when new. What you think? - Glenn
I suspect that enlarging clearances of various engine parts due to wear is an important contributor to increased noise as engine ages. I'd guess that piston/cylinder clearances are most to blame. Your list above sounds reasonable too.
In addition, I think drying, aging rubber and plastic parts (mounts, seals, gaskets, trim) lose their ability to dampen noise, and they change their resonances as they get stiffer or deformed. Have you noticed your car's suspension feeling (and sounding) more supple in the rain? I think this is due to the lubrication of rubber parts by water, an effect not as noticable on a new car with new rubber parts. Also, my car's interior trim sounds different (buzzier) when it's colder and thus, stiffer. (I don't think it's my imagination.)
When you put interior trim and audio equipment aside, all cars are subject to three different sources of vibration: 1st order: This involves noise/vibration from both the engine and accessories. In most cases the vibration or noise will get progressively louder as engine speed increases. This could be due to internal engine wear, accessory wear, belt wear and other things you've suggested. 2nd order: These vibes involve the drivetrain. This includes transmission, driveline and differential components. Usually noises/vibrations will either coincide with vehicle speed or a certain RPM range. 3rd order: This deals specifically with axles and the wheel/tire assemblies on the vehicle. Noise is always related to vehicle speed and usually most appearent at highway speeds. All the things you suggested can cause engine or other noises from your vehicle. The climate changes vehicle's interiors are exposed to also cause the plastics to expand and contract, causing mounting points to become loose and cause rattles etc. Anyways, I hope this helps some.
almost all engine noise that you hear is from the valvetrain. i have been to a race shop where they had a spintron machine. a spintron letst you test engine valvetrains. you take the crank/rods/pistons out of a block and hook up a very fancy and expensive DC electric motor and run the engine to whatever rpm you want to torture test the valvetrain. the noise that engines undergoing this is 90% of what you would hear out of them on the track. only the exhaust boom is gone.
That is very interesting jamesn. It's exactly the type of thing I was trying to learn with this thread. Thanks for the info. - Glenn
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