Why is there a hissing air leakage sound at idle?

[Sauron]

Engine hissing sound due to air leakage, generally after hearing this sound, the engine will rapidly lose power. Possible causes include engine overheating, requiring inspection of the cooling system, or exhaust system blockage, vacuum tube leakage. Below are the relevant explanations: When the engine is running, a "whooshing" sound heard at the carburetor air intake or air filter indicates intake valve leakage. A "hissing" sound heard at the exhaust pipe or muffler indicates exhaust valve leakage. The reasons for valve leakage include the following: 1. Wear and corrosion on the working surface of the valve and valve seat, producing spots or depressions and accumulation of carbon deposits. 2. Excessive clearance between the valve stem and valve guide sleeve, causing the valve stem to wobble, preventing the valve from closing tightly, or bending deformation of the valve stem, tilting of the valve head, resulting in leakage and producing a "whooshing" or "hissing" sound. 3. Weakened or lost elasticity of the valve spring, or spring breakage, causing improper sealing between the valve and valve seat. 4. Insufficient valve lash clearance, causing the valve stem to expand when heated, and the tappet or rocker arm to push the valve open, preventing the valve from fully closing and resulting in leakage sounds.