Some of the 4 and 6 cylinder engines have had the problem of the crankshaft pulley bolt coming loose. This will make a rattling noise as the engine idles. Our technicians tell us that the pulley bolt should have a thread locker applied and then retorqued.Google+
Car Problem Reports
Toyota 4Runner Crankshaft Pulley Bolt May Come Loose
Toyota 4Runner Problem
Engine Affected: 3.4L V6
Average Mileage: 88,500 mi (81,000 mi - 96,000 mi)
Visitor, 1997 Toyota 4Runner, 3.4L V6, 96,000 mi
Bolt on crankshaft pulley became lose, allowing the pulley to move a bit. It eventually allowed the crankshaft to core out the inside of the softer pulley until a press-fit was no longer possible. This damage happened before I bought the care used and was not aware until it worked lose again and threw the belts.
New bolt and junkyard pulley were installed, good press fit still possible even though crankshaft was worn. Still fits tight 70k miles later and one timing belt replacement. Before doing it myself the dealer recommend a bit of locktight on the bolt and shaft, but did not need it.