Q: My car is making a loud rattling sound, but only when it's cold and I've just turned the engine over.

As a car sits, all the residual oil (in the hydraulic valve lifters, for example) drains down to the oil pan. When you re-start the car, particularly when the weather is cold and the oil is most viscous, the valve lifters and all the other rotating engine components have to be re-supplied with oil under pressure. The problem is that, until the oil system reaches full pressure, there is the potential for noise, including what might sound like rattling noise from the valve train.

If you suspect the noise could be coming from belt driven accessories, such as the alternator, water pump, power steering pump, and so forth, you can disconnect the serpentine belt from the engine and run the engine just briefly (as before, that is after it has sat idle and is cold) and see if the noise disappears. But, more likely than not, the noise is simply due to that brief interval when your engine lubrication system is not fully pressurized.

There are aftermarket engine pre-lube systems that you can add to your car to avoid hearing even this momentary noise. This pre-lube system is operated via a solenoid that is wired into your ignition switch circuit so that the instant you turn the key to the "on" position (before "start"), a valve opens from a small pressurized cylinder containing oil, instantly creating oil pressure throughout the engine even before you start your car.

I recommend having a certified mechanic, such as one from YourMechanic, come to your location to determine why the car is making a noise and suggest any needed repairs.