What Causes the Buzzing Noise at Fixed RPM?

[SanMargaret]

The causes of buzzing noise at fixed RPM are as follows: 1. Noise occurs only at idle or low speed: Excessive clearance between piston and cylinder wall, overly tight piston pin assembly or connecting rod bearing assembly, excessive clearance between tappet and its guide hole; 2. When maintaining a certain RPM, the noise becomes chaotic, and brief sounds are emitted successively during rapid deceleration: Cracked camshaft timing gear or loose fixing nut, broken crankshaft; loose piston pin bushing; 3. The noise appears during rapid engine acceleration and persists at high-speed operation: Loose connecting rod bearing, melted or mismatched bearing shell causing rotation, loose crankshaft bearing or melted bearing shell, broken piston pin, broken crankshaft.