1991 Nissan 240sx - Strange noise from underneath car

There is a grating noise coming from underneath my car that is getting progressively worse with time. It is fairly loud, especially after I have just started my car. The sound tends to go away as I get to speed and usually doesn’t return until the next cold start. It is not a repetetive sound. I think of a marble getting slapped around in a tin can. Any idea what this could be, as well as any explanation why it seems to get quiter and go away as I drive.

This sounds (no pun intended) like the classic loose/bent heat shield problem that many people experience as their cars age. This is not dangerous, and is only an annoyance.

The next time that your car is in the shop for an oil change, ask the mechanic to try to clamp or screw the heat shield(s) securely, and your problem should be over.

Thanks a lot, I’ll look into that. Is the heat shield attached to the transmission or something that causes it to vibrate during the first minute or so after startup?

The heat shield(s) are placed on or above your exhaust system in order to protect other components from the extreme heat of the exhaust system. When your engine is in its first few minutes of warm-up, it does not run quite as smoothly as it does after it has warmed up. This slight roughness from the engine is transmitted to the exhaust system, and this is transmitted to the heat shields–ergo, vibration.

On some cars, a particular range of engine RPMs can cause heat shield vibration even when the engine is warmed up. In other words, on some cars, you might get heat shield vibration between…let’s say…3,000 RPM and 3,500 RPM, but not at lower or higher engine speeds. So, if you only have this problem during warm-up, you have a less of a problem than some other drivers.

Heat shields can be an annoyance, but don’t succumb to anyone who says to just remove them in order to eliminate the noise. Car makers are very cost-conscious, so there had to be a good reason for them to spend the money to install them on the car in the first place. Just get the heat shield(s) secured properly and the problem should disappear.