I have a 2005 3.8-liter Chrysler Town & Country with about 152,000 miles on it. It has a constant engine noise like a diesel pickup truck when parked and idling.

The Chrysler dealer was not sure what was making the noise and suggested they start by replacing the generator or water pump to see if that solved the problem. I decided against this trial-and-error method, took the car home, and kept driving it.

It has been months now, and neither the generator nor the water pump has failed. I seem to remember an article you wrote that suggested checking the timing chain/belt and tensioning sprocket. Could this be the issue?

Answer: Since the vehicle makes this noise while idling in park, a technician should try to pinpoint the source with a mechanic's stethoscope. By touching the tip of the tool's pick-up to belt-driven components like the alternator, water pump, and power-steering pump, then the timing cover, valve cover, cylinder heads, exhaust manifold, block, etc., there's a very good chance of pinpointing the noise.

Q: I drive a 2006 BMW Z4 with only 25,000 miles on the odometer. I intend to keep the car indefinitely, and I try to keep it in excellent condition. I also drive a 2014 BMW X1 with the M package and four-cylinder turbocharged engine.

Are there advantages to using ethanol-free gasoline in these cars? I'm aware owners of older classic autos use it and wonder if it would benefit me.

A: Both your vehicles - and virtually all vehicles built in the last two-plus decades - are engineered to deliver good performance, mileage, and longevity operating on ethanol-blended fuels. Strictly speaking, there should be no advantage to operating on non-ethanol fuels.

Perhaps more important, in many states ethanol-free fuel is often sold for specialized equipment such as collector cars, boats, off-road vehicles, and small engines.

But if you were able to ask your vehicle which fuel it prefers, I suspect it would choose 100 percent gasoline. Theoretically speaking, the advantages of operating a late-model vehicle on non-ethanol fuel would be slightly better fuel economy, less chance of fuel-system deterioration and phase separation of water and fuel, particularly in vehicles operated infrequently.

These small benefits are offset by significantly higher costs, potential legal issues, far fewer locations to purchase, and the compatibility of ethanol-blended fuels with today's vehicles.

Q: I have a 2004 Volvo XC90 with 275,000 miles that still serves as a daily driver. It runs great, but at this mileage I can't find much meaningful maintenance guidance online.

In regard to oil viscosity, I decided to try 20W-50 last summer, with pretty good results - I got almost 3,000 miles before a quart was needed. The thinner oil won't quite go that long anymore. Any reason not to go with 20W-50 or synthetic 15W-50 on a car with this many miles?

A: I'd stick with the 20W-50 conventional oil because the synthetic costs considerably more and may generate higher oil consumption due to its uniform rather than random molecular size.