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Car Knocks

My 2003 Nissan Maxima has a knocking at around 45 mph, even with hightest gas, which I am forced to use in this car. I have 137,000 miles on it…its a great car!

Is it a knock or a ping? Pinging would be affected by octane, knocking wouldn’t.

Sorry…I don’t know the difference. It goes away when I accelerate.

If it was pinging, that would occur during acceleration, not when driving at a steady speed.
A genuine engine knock would tend to get louder and more frequent when accelerating.

Ergo–I don’t think that your choice of gas has anything to do with the problem, and I don’t believe that this has anything to do with the engine. However, I reserve the right to change my answer once we get more complete information.

The complete information that we need includes the OP localizing the noise for us, since we can’t hear it.
What area of the car is it coming from?

We also need a better description of the knocking.
Is it a “tinny” sound?
Does it sound like someone beating on a bass drum?
Does it sound like two sticks hitting each other?

Does it make the knocking noise once, and then does it stop?
Does it knock repeatedly?

Is it related to engine speed–i.e., does the frequency get faster as the car goes faster?
At–let’s say–60 mph, how many times per minute do you hear the noise?

Does it make the noise when idling, or only while driving?

You have to give us a lot more information, as currently your question is the automotive equivalent of phoning your doctor, and saying, “I have a pain somewhere in my body. What is wrong with me?”

Then it is definitely a “ping”. The two sticks hitting togher sounds right.It doesn’t get faster as car goes faster. It only does it when accelerating from say 35 mph to 45 mph. Then it stops. No noise when idling. Thanks!! Sorry it is difficult. It is more like a “rattle”.

This sounds like a pre-ignition rattle and in most cases this is caused by an EGR system fault.
You might get the car scanned for codes to see if any EGR codes are present. AutoZone, Checkers, O’Reillys, etc. will do this for you free and it only takes a few minutes.

It’s also possible to have an EGR system fault with no codes present at all. At 137k miles it could be that the EGR passageways are clogged up and need to be cleaned out.

Okay–now I’m really confused.
First, you stated, “It goes away when I accelerate”, and now you tell us, “It only does it when accelerating from say 35 mph to 45 mph”. Is it just me, or are those two statements inconsistent?

Anyway–in the hope of finding a solution despite little useful information, I suggest that you have your mechanic check the exhaust system.
I suspect that you have a loose or damaged heat shield on some part of the exhaust system.

Depending on exactly how loose a heat shield is, or how it is damaged, these pieces of sheet metal can produce harmonic vibrations at certain RPMs, but not at other RPMs. The noise can be alarming, even though it is not dangerous.

Ok, Thanks! I have no idea what EGR is, but guess mechanic will. Thanks.