I have a 2000 Nissan Maxima, and when I go over a speed bump it makes a “thump” sound when it lands back on the road. The sound comes from the rear, as if something heavy is loose in the back and that makes the sound when it lands after the vehicle comes down from a bump. I have checked the trunk, and everything in the trunk is strapped down. I have taken the car to a few shops, but they have not been able to identify the problem. Can you help me identify the issue and suggest what I can tell a shop to look for?

Well, it sounds like you’ve already checked for bowling balls and mothers-in-law rolling around in the trunk, so I’m all out of ideas.

Actually, this is a hard problem to solve without having the car in front of me. But based on your description, my first guess would be that your exhaust system is banging against the underside of the car.

Since your car’s been around for more than a decade, I’m guessing that at some point, you replaced the exhaust system. And if you got an aftermarket exhaust, it might not fit as precisely as the original did.

That doesn’t mean it’s not doing its job; it’s just that exhaust systems are supposed to bend and curve and rise and fall to fit the exact contours of the underside of each car. And if one of the bends isn’t exactly right, that can bring the pipes too close to the bottom of the car, and allow it to bang against the undercarriage when the suspension fully extends or compresses — which is what happens when you go over a speed bump.

Another possibility is that one of the exhaust hangers — the hanging, rubber doughnuts that hold the entire exhaust system in place — has fallen off. That sometimes happens. And that can allow any portion of the exhaust pipe to clank against the bottom of the car, too.

There are other things it could be: A worn-out or broken McPherson strut mount comes to mind first. But it also could be something like a loose sway bar or a bad sway bar mount. Those things tend to make rattling noises more frequently, on bumps of all sizes, not just speed bumps. But I wouldn’t rule them out.

So next time you take your car in for an oil change, ask the mechanic to look at your rear suspension. Maybe he’ll see something obvious that’s worn out. And have him look at your exhaust system, too. He may see evidence that a pipe’s been hitting the underside of the car somewhere. Sometimes you can see where it’s been scraping.

But if there’s not an obvious fix, like a missing hanger, at least consider leaving it alone. Every time we try to adjust a poorly fitting exhaust system for a customer, we always seem to make it worse. So you’ve been warned. Good luck.

I was wondering if there is a way for gas stations to add water to their tanks to make the gas stretch farther. I heard differing points of view about this when I posted this question on a popular social-media Q-and-A forum. Some say gas and water don’t mix, so it would be impossible; others say it would destroy the machinery involved, as it wasn’t made to handle water; others say it’s very possible. What do you say?

It’s possible. And it’s also impossible. More to the point, it’s possible for gasoline and water to mix temporarily.

If you shake up a container of water and gasoline, the water will be briefly suspended in the gasoline, but will quickly separate back out, with the water going to the bottom and the gasoline staying on top.

That’s why this trick is popular right now only with gas stations in Oklahoma, where they have about 5,000 earthquakes a month, thanks to fracking. That keeps those underground storage tanks shaken up nicely.

So the overall answer is no, it does not make sense for gas stations to do this. The water would sink to the bottom of their storage tanks, taking up space, and reducing the amount of gasoline they could store and sell.

And since the water would separate out, some customers would get only water, which would leave the gas station with a lot of ‘splainin’ to do.

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