If you are hearing a strange sound coming from under-the-hood of your vehicle, don't press your luck and ignore the issue. Often times, these sounds serve as a warning that you should heed as it could indicate something seriously wrong with your engine and prompt attention may save you from being stuck on the side of the road later on.
The sounds that you hear can help you identify what is going on with your vehicle:
Low or no oil in the engine. When your vehicle has low or no oil in the engine, the sound is loud and harsh. It almost sounds like your vehicle is straining to run, which it may be as the engine gets very hot when the oil is low.
Issue with the battery. You have probably heard the sound that a bad battery makes when you try to start your vehicle. The ticking sound is usually followed by the inability to turn over the engine, which might be resolved with a jump. Bad cells in the battery also make a ticking sound, though a jump will not resolve this and the battery should be replaced.
Slipping transmission. A slipping transmission may sound like it is in the wrong gear. Your engine will sound like it is revving up, and struggling to get in the right gear.
Problems with pistons. If you have piston problems, you may hear a slapping, flapping sound coming from under the hood. Talk to your mechanic about faulty pistons and repair estimates.
Engine valve failure. An airy, loud tapping sound could be indicative of issues with your engine valves. The sound is faster than the tapping of slapping pistons, and the valves have a metallic sound.
Adjust your lifter. The pinging sound when your lifter needs adjustment is fast and loud. Take it to your mechanic to get a lifter adjustment.
The Engine is seizing. The sound of an engine seizing is hard to miss. It is loud, hollow, and quite unsettling. Don't try to operate your vehicle, even if it does seem to run okay.
Steering fluid is low. When your vehicle is low on steering fluid, you will likely hear a loud squeal when you make sharp turns. This may level out after driving for a while, but you should check the reservoir and fill your fluids.
The sound that your engine makes can help you to diagnose exactly what is going on with your vehicle. For a more thorough and precise assessment, take your car or truck to a qualified mechanic (such as one from Auto & Truck Used Parts) that you trust. They may be able to identify the issue, and resolve it before you find yourself stranded on the side of the road.