Loose Flyweel Bolts
Flywheel’s rotate at very high speeds with weight and mass that absorbs the engine pulses transferred through the crankshaft. They’re balanced and must be placed in the same position as they where removed. The bolts are torqued in a crisscross pattern according to the manufacturer's specifications. If the flywheel’s bolts are loose, a rattling or knocking sound that’s affected by engine speed and can be heard under the engine, where it mounts to the transmission bellhousing.
Flywheels are installed on vehicles with manual transmissions and flexplates are installed on vehicles with automatic transmissions. This is because the mass of a torque converter used with an automatic transmission acts like a flywheel, smoothing out engine pulses. Flywheels and flexplates contain a starter ring around the outside of their circumference. They are integral to flexplates but can often be removed and replaced on flywheels.
A loose bolt on a flexplate or a crack between it’s holes can result in the same knocking or rattling sound in the same area a loose flywheel produces. If there’s a rattling or knocking sound after an engine has been removed and replaced, the flywheel or flexplate may be installed incorrectly or the bolts may be loose. If possible, remove the small torque converter inspection cover and inspect the part closer.