engine sounds like a Tractor
I remember reading about this in another thread on this site. I am not able to find it today, but the owner found out that his catalytic converter was getting clogged up. Its an easy check at a muffler shop. They drill a hole somewhere close to the beginning of the exhaust, engine side, stick a meter in the drilled hole, and monitor the backpressur of your system. A matter of seconds will show stopped up or not.




I'm no expert but pre-detonation could be due to low octane fuel, bad fuel, a firing issue, or even a clogged cat, as suggested by sharp2177.
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Is the acceleration or performance less than before you noticed the sound?
If performance seems the same with no hesitation or bogging down, then I think it could be an exhaust related issue. If there is a leak and it is on the underside of the tubing you might see a small water pool under the car; other than that just aft of the oil pan area when you run your A/C. If the exhaust is stock however, I think it is unlikely that there is an exhaust leak.
Do not discount a defective cat and or downstream blockage due to dislodged pieces of the cat core. A reputable muffler shop of indy should be able to eliminate the exhaust fairly quickly, as pointed out by someone else.
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I would make all easy checks first. You are on the right track by checking possibilities online before throwing a lot of cash at the problem first. So, I would check the exhaust pressure as an easy check, but check fuel grade and other easy checks mentioned in this thread also. My CLK500 has run great so far with 135000 miles. But I keep up with these threads to keep informed and possibly save time and money later.




