What causes the buzzing sound when the car is just started?

[DeBenjamin]

There are three reasons for the buzzing sound when the car is just started: 1. Engine belt issue: Check if the tightness of the engine belt is appropriate. When checking, use your fingers to pinch the belt and twist it. Note that this operation should be performed when the engine is cool, and the twist angle should generally be below 90 degrees; otherwise, adjustment is needed. 2. Engine compartment abnormality: Sometimes, foreign object blockage can cause the vehicle to squeak. 3. Brake pads: If the buzzing sound occurs when braking while driving, it indicates that the brake pads have worn to the limit and have started to grind against the brake disc. In this case, you should go to a repair shop for inspection as soon as possible.

[McFaith]

My old car also makes this kind of squealing noise during cold starts, which usually lasts for a few dozen seconds before disappearing. The most common cause is an issue with the engine belt system. Over time, belts can become aged and hardened, developing fine cracks on the surface, or their friction can decrease due to oil contamination, resulting in a sharp rubber friction-like sound during startup. Incorrect belt tension can also cause noise—too loose and it slips, too tight and it accelerates wear. Another possibility is that the bearings in one of the accessory components are starting to fail, such as the alternator, air conditioning compressor, or power steering pump bearings, which can produce such noises when worn. I recommend opening the hood during a morning startup to carefully listen for the source of the noise. If it's confirmed to be the belt, you can first try cleaning the belt grooves. If the noise persists, it's time for a replacement—a new belt costs around a hundred bucks and can even be replaced yourself.

[StKai]

As someone who frequently helps friends with car repairs, I've encountered many cases of abnormal noises during startup. If the noise resembles metal friction with a slightly sharp tone, prioritize checking the alternator. When the carbon brushes inside the alternator wear to a certain extent, poor contact in cold conditions can produce a sizzling electrical corrosion sound that disappears when warmed up. Another possibility could be the sound of refrigerant flowing through high-pressure pipes in the air conditioning system, but this typically occurs after turning on the AC. A leaking brake vacuum booster pump may also produce a brief suction sound at startup, but it won't last long. The easiest identification method is: start the car in a quiet garage and have a companion listen for the noise location outside the vehicle - the alternator is at the front right side, while the booster pump is near the firewall. I recommend focusing on testing the alternator's power output first; bearing failures can be replaced individually at low cost.

[MacKendra]

The buzzing sound during ignition can indeed be unsettling, but it's likely a belt issue. The friction noise from a slipping belt is like nails on a chalkboard, especially noticeable in winter when rubber hardens. I've experienced it myself—replacing the belt instantly restored quiet. However, if the sound resembles an electrical buzzing, the fuel pump might be suspect. The fuel pump in the tank produces a faint electrical hum during operation, usually inaudible through the tank; if abnormally loud, the pump may be failing. Another often-overlooked cause: poorly grounded aftermarket audio systems can emit electrical interference noise through speakers during startup.