When my car is coasting or in park, I can hear a very loud rattling sound coming from underneath the vehicle. The sound is not always constant, as sometimes it happens every 15 to 45 seconds, and sometimes it is almost constant.
This could be caused by a loose lower cover plate. Trucks have a metal plate that protects the underside of the engine from debris. This plate is often removed for repairs and sometimes not all the bolts will be present as they tend to come loose from the cars vibration. This can cause a rattling sound under the vehicle. A certified mechanic will be able to pinpoint the source of the noise, and check the bolts for tightness and make sure none of the bolt holes are damaged. So that new bolts can be installed if some are missing.