Reader is all out of ideas about mystery noise

Dear Car Talk:

In hot weather, I occasionally hear a very loud horn-like sound when I drive at 65 mph or higher. The noise appears to be on the passenger side, toward the front of the car. The local Honda dealer and a local mechanic have checked belts, windows, sunroof and the hood latch. It seems to happen only when I am driving on a curve, and it will stop if I slow to less than 65 mph. I need your help. Thanks. - Maggie

Ray: Well, since you've had several mechanics look at this, I'm going to make two assumptions. Well, three, actually.

Assumption One is that the mechanics have heard the noise. Assumption Two is that once they’ve heard the noise, they’ve ruled out something dangerous, like a failing wheel bearing, and they’re convinced it’s more likely a wind noise - which explains the stuff they’ve checked so far. And Assumption Three is that, since two shops have already been all over the car, I have no chance of figuring it out.

Back in the old days when a customer had a problem like this, we’d strap my brother to the hood of the car and then go for a drive and try to reproduce the sound. When we got back to the shop, we’d take the leaves out of his mouth, and ask him which direction the noise was coming from.

There’s a more modern-day version of that: We have a tool at the garage that has a bunch of small, wireless microphones that attach with hook-and-loop tape straps, suction cups or magnets. With that tool, we can attach a bunch of sensors to various parts of the car and then drive the car, tuning in to one mic at a time. That allows us to home in on the exact location of the sound. That in itself doesn’t solve the problem, but it at least narrows it down and gives us a fighting chance.

My first guess on something like this - although it’s just a guess - would be a bad windshield seal. As the speed of the car increases, the body flexes more. And when you make a turn at 65 mph, the windshield opening may be flexing just enough to move the windshield gasket or seal and create a wind noise.

So here’s what I’d do: First, I’d ask your dealer to confirm that he’s ruled out anything dangerous, like a wheel bearing. Then I’d look for a shop that has one of the listening devices I mentioned earlier. Like NSA Motors.

And then try out some theories. Put a couple of microphones around the perimeter of the windshield, and see if the noise is very loud near one of them. Or you can do it yourself by duct-taping your iPhone on there … as long as you have the dropped-phone replacement coverage. Good luck, Maggie.

Can gasoline ‘go bad’ in hybrid engines?

Dear Car Talk:

If you drive one of the new plug-in hybrids or a gas/electric hybrid on pure electric for an extended period - say, six months or so - is it possible for the gasoline in said hybrid to go "bad"? I'm guessing not, that the movement of the car would keep the fuel mixed and not let it separate (or whatever happens to old fuel). Thanks! - Ed

Ray: Theoretically, the gasoline can go bad. Condensation can build up in the gas tank, and if gasoline sits long enough, it can break down and create varnishes, which can clog fuel injectors.

But manufacturers have anticipated the problem. In the Chevy Volt, for instance, if you’ve been driving on nothing but electric power for six weeks, the car will go into maintenance mode and run the engine for a short period of time. That removes vapors and condensation, and keeps the engine lubricated.

If you manage to go a whole year without using the gasoline engine, the engine will automatically run until the tank is empty - or until you refill it with fresh gas.

Toyota has a similar system for its plug-in hybrids.

Bottom line: Don’t worry; be happy, Ed.

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