Dear Car Talk:

I have a 2008 Nissan Altima Hybrid with 93,000 miles. Beginning last winter, within the first couple of minutes of starting the car, there would be an intense shaking and rattling noise under the hood. It's still happening, especially when the car hasn't been used for a couple of days. This only occurs when the car is started for the first time. For the rest of the day, the car operates normally.

I had my mechanic look at the car, and he thought it was the engine mounts. So, the mounts were replaced. That didn't fix it. I brought the car back, and the mechanic experienced the problem but said that it happens for such a short time that the diagnostic system couldn't produce a code. Any ideas what could be causing this and how it can be fixed? -- Spencer



If it's not producing a code, that suggests it's something mechanical, rather than electronic. And if I had to take a wild guess -- which is usually what I do -- I'd guess it's your harmonic balancer. Every gasoline engine has a harmonic balancer, not just those sold in California.

The harmonic balancer is a big pulley that sits on the front of the crankshaft. It's made of two concentric metal discs with a piece of rubber between them. And its job is to damp the vibrations created by the engine's crankshaft. If your harmonic balancer is slipping when it's cold, it could create lots of vibrations when you first start the car. And then once it warms up, it may start working properly. And that would not set a code.

If you had a bad injector (which would be my second wild guess), a bad spark plug or a bad coil, that probably would have a set a code that your mechanic would have found. So drop off the car with your mechanic some afternoon. Have him put it up on the lift and leave it there overnight. And when he starts the car the next day, he should have someone under the car to specifically watch the harmonic balancer and see if it's wobbling or shaking for the first 30 or 40 seconds. See if it stops vibrating after that. If your mechanic catches it in the act, he'll know what to do next, Spencer. Good luck.