•Q. About three weeks ago, I was pulling into a parking space, and just before stopping I heard and felt what appeared to be a large impact. There was no one around and nothing to hit. I walked around the car and looked under it, but I could see no problems. Today I was pulling into a parking space and heard and felt what appeared to be an impact from another car. There were no cars around, nor objects that I could have hit. I did a walk around, but I could find nothing wrong. Both times, my dogs were in the car, and both times they became agitated. I have tried to reproduce the effect, but could not. If it had happened once, I could ignore it, but twice suggests that it will happen again. The next time, it could be at a higher speed. The first time, I was within a few feet from stopping, and I had just turned to the left. The second time, I was a few feet from stopping, and I was turning to the right. The only thing the two incidents had in common was that I was nearly stopped. My limited knowledge of cars leaves me puzzled. Can you help?•

RAY: Our limited knowledge of cars often leaves us puzzled, too.

TOM: We should start by saying that we don’t know what’s wrong with your car. We’ll give you some ideas, but this is something that a mechanic is going to have to find for you using his eyes, ears, hands and tuchus.

RAY: Whenever there are strange noises or, more importantly, shaking coming from the front end, there’s one thing we always check first:

TOM: Whether the customer is up to date on his liability insurance premiums.

RAY: Right. Because shaking and quaking from the front end can mean that an important piece of your front suspension or steering assembly is worn out, which means a wheel could fall off. You don’t tell us the age of your car. But obviously, as a car gets older, chances of this sort of catastrophic failure go up.

TOM: So, ask a mechanic you trust to take a thorough look at the front end. Make sure your ball joints, wheel bearings and control arms, etc., are all in good shape and still firmly attached to the car.

RAY: If they are, my first guess would be that you have a front axle that’s binding up. That tends to happen when you make the sharpest turns — like when parking.

TOM: The next thing I’d look at would be your motor mounts. If you have a broken motor mount or two, your engine and transmission literally can jump around inside the engine compartment. And under certain circumstances, it could create jolting sensations as it jumps into or out of position.

RAY: You also could have a brake that’s sticking. You’re obviously using the brakes when you park, and if one of the calipers gets stuck, it could jolt the car when it gets unstuck.

TOM: The final thing to suspect would be the transmission. It could be something as simple as a rough downshift into first gear that you’re feeling. But it would have to be awfully rough to do what you describe. And I can’t explain why it would happen only when you’re parking.

RAY: So those are the things to start with. Get the potentially deadly stuff checked out first, and then move on to the merely obscenely expensive stuff. Good luck!

•Q. I have a 5-gallon gas can that has an old-style spout, which used to have a bright-yellow cap. The cap got lost, so I got a rubber stopper to put in the end, to prevent vapors from escaping when not in use. Well, last night I put some gas in the car from this can, and not thinking, I forgot to take off the rubber stopper. You guessed it, the gas did not pour, but with a little shake, it started to flow, and a lightning bolt hit me: “Oh *#!! — the stopper fell in the gas tank!” My question is, What to do now? Will the stopper dissolve and mess up the fuel injection? How long would that take? Could the stopper roll around and block off fuel to the pump?•

TOM: You can go back to sleeping at night. I doubt the stopper’s going to hurt anything.

RAY: One of two things will happen. Either that stopper will just sit at the bottom of the gas tank forever, never bothering anybody, or it will slowly disintegrate.

TOM: Some types of rubber, like neoprene, can stand up to petroleum products. Some can’t. I have no idea what the chemical makeup of your stopper is. If it was a stopper designed for a gas can, I’m sure it’ll just be an innocuous, permanent resident of your tank. Whereas if you pulled it out of a bottle of Baboon Thigh Pinot Grigio, it might break down over time. But I still doubt it’s going to cause any problems further upstream.

RAY: Even if the stopper disintegrates slowly and gradually dissolves in the gasoline, the small number of dissolved rubber molecules in any given tankful of gas probably will just combust in the engine, along with the gasoline, and never be noticed.

TOM: And if the stopper dissolves into small rubber bits (which probably is more likely), and those pieces sink to the bottom of the tank, they’ll be prevented from entering the fuel line by the “sock” filter on the bottom of the fuel pump.

RAY: And if, by chance, some very, very tiny bits get through that sock filter somehow, most cars have a second, multi-micron-level filter further upstream to catch those even-smaller impurities and protect the fuel injectors.

TOM: So the car companies obviously have dealt with people like you before. And they were ready for you this time. I think you can sleep easy.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.