1998 Dodge Durango just bought 3 weeks ago

A loud knocking sound is commonly a worn or "spun" connecting rod bearing. Typically the engine needs to be rebuilt and the cause of the loss of oil pressure determined. If the oil level is full, the spun bearing would cause low or pulsing oil pressure. No oil in the engine, or a plugged or broken oil pump can lead to the spun bearing(s). Either one can cause the other.

This should be listened to and inspected by a professional since there are other causes of noises.

You'll need to get estimates for an engine rebuild or used engine. I bought a Caravan with this problem. Parts, including a used crankshaft and a fairly expensive gasket set cost $300.00. Students took care of the labor to remove and install the engine and to perform the machine work, but this was a six-week project. To pay a shop to do the work in a timely manner, expect around $2,000 to $3,000, possibly less if they install a used engine.

Caradiodoc