Q: I have a 2003 Pontiac Vibe, which is basically a Toyota Matrix, with 135,000 miles on it. This past winter, on a cold start it would knock for a minute or three, depending on how cold. A friend who is a GM technician said that it is a cold piston slap and it will last for a long time. Now, because I am aware of the sound, I can faintly hear it this summer. Could it be anything else?

A: The real concern would be if the knock came from a crankshaft or rod bearing. But I don’t think the knock you’ve described is from the bottom end of the engine. Usually, a bearing knock on a cold start lasts only a few seconds until the oil pressure comes up.

Piston slap, on the other hand, does tend to last longer until the piston heats up, expands slightly and reduces the clearance between piston skirt and cylinder wall. In some cases, like my 42-year-old Corvette, the noise never goes away due to age, wear and mileage. But a piston slap is typically not harmful, nor does it shorten engine life. It’s just annoying.

Which is it in your vehicle? Use an inexpensive mechanic’s stethoscope or long wooden or metal rod to “listen” to your engine. Place the end of the “scope” against the side of the block near the top of the cylinders, then hold the other end to your ear and listen carefully. Do the same thing at the bottom of the block just above the oil pan. If it’s piston slap, you’ll hear it at the top of the block. If it’s a bearing, it will be loudest at the bottom of the block. I’m betting on piston slap.

Q: I have a 2001 Cadillac STS with 136,000 miles. One high-intensity discharge, or HID, headlight is out. The same thing happened four years ago, and the cost of replacing the headlight and ballast unit was $1,500. Do you have any suggestions that would be less expensive?

A: Start by checking the 15-amp fuse for the nonworking headlamp located in the underhood fuse block. If it’s good, try swapping the working HID bulb, referred to as a starter/arc tube, for the one that doesn’t work. If it lights, the problem is the D1R HID bulb. If it doesn’t, the ballast unit is likely the problem.

I searched online and found the D1R bulbs for about $30 and the ballast unit for around $90.