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Q: I have a 2008 Chevy Malibu with 72,000 miles. Last week I heard a rattle on the driver's side, the same side where the exhaust pipe is located. The rattle doesn't happen above 5 to 10 miles per hour; it happens while stopped or backing up my driveway. The dealer says a piece of metal broke off inside the exhaust muffler. They can't get to the problem without replacing the whole exhaust system for $700. They said it won't be a problem for the car itself. What do you think?

A: In my experience, this type of exhaust rattle is often caused by a broken or cracked heat shield surrounding the bottom of the catalytic converter. I've seen the shield repaired by welding or wrapping with a steel shipping strap. I've also seen the entire heat shield removed, although this exposes the extremely hot catalytic converter to potentially combustible materials like leaves and newspaper.

I'd suggest getting a second opinion, perhaps at a muffler shop. Ask the technician to check the converter shield. If it is not broken and there actually is broken metal inside the muffler, I see no reason the muffler itself cannot be replaced.

One more thing. The federally mandated emissions warranty covers the catalytic converter and computer for eight years/80,000 miles. If the rattle is coming from the converter itself, it may be covered by the emissions warranty.

Q: I bought a new 2013 Ford F150 that now has 6,600 miles on it. From day one, it has clunked when I accelerate from a stop. The dealership installed a shim kit in the rear end, greased the U-joints and said the clunk was gone. It isn't gone. It's worse, if anything. A Ford field representative said this was perfectly normal and was engineered to do this. Do you think this is actually the case? I didn't spend $40K to have to live with an irritating problem that makes the truck feel like an old clunker.

A: That's a tough one. I looked up Ford technical service bulletin No. 14-0090, dated May 2014, in my ALLDATA database. It describes an "intermittent click or snap-type noise from the rear axle on initial light acceleration from a stop while in drive or reverse." Two repairs are outlined. If the first repair — applying a Motorcraft penetrant to each of the U-joint retaining clips — does not stop the clunk, the second step outlines the procedure for removing the pinion gear flange, replacing the oil slinger, adjusting the pinion bearing pre-load and reassembling the flange and U-joint coupling with four new bolts.

It would appear the dealer has performed the second-step repair. Since the vehicle is still under warranty and the repair was not successful in stopping the clunk, I would suggest you encourage the dealer to perform the repair again. Unless Ford develops an updated repair for this issue, your options are limited.

Q: At the gas station, if the car before me was refueled with 87 octane and I select 92 octane, how much 87 octane fuel is pumped into my gas tank before the 92 octane fuel is delivered? Also, I've read that there is no difference between 91, 92 or 93 octane fuel; it is just a difference in how the octane is calculated. Is this true?

A: Interesting question. My neighbor, who like me uses non-oxy gas in his boat, tells me that as much as half a gallon of the previous octane choice will be pumped before your octane choice begins to fill your tank. I don't know if this is accurate, but even if half a gallon of 87 octane is added to 15 gallons of 93 octane, it won't significantly affect the octane level reaching your engine.

In the United States, the octane rating shown on the pump is the fuel's "motor" octane number (MON), developed from a test engine under load, added to its "research" octane number (RON) from a laboratory engine, divided by two.