Loud Rattling or Slapping Noise from Engine on Toyota 4Runner

Loud rattling or loud slapping may be heard from the engine on the 1996-2002 Toyota 4Runner with the 2.7L I4 engine.

These noises has two known causes: a failed balance shaft bearing, and a failed timing chain tensioner.

The balance shaft, also known as the counter rotating assembly, is installed to reduce engine vibration, and counter the rotational force of the crankshaft and camshafts. When the bearing fails, it will rattle loudly, and the timing chain will produce a metallic slapping noise. The rattle has been described as shaking a can of marbles.

The timing chain tensioner is also known to fail, allowing the timing chain to contact the timing chain housing. This produces a metallic slapping noise coming from the front of the engine, but there is no "can of marbles" sound associated. This would not indicate balance shaft failure, but the timing chain may need to be replaced.

To repair these issues inspection of the entire timing chain system will be necessary, and replacement of the timing chain, timing chain guides, balance shafts, timing chain tensioner, and associated seals may be necessary.

Avoiding these issues may be possible by early oil changes, and using the factory specified engine oil.