oil pressure gauge on 2003 Ford Expedition

I just went on a 500 mile trip. All at once the oil pressure bottomed out and the light came on. I pulled over at the next stop and topped off the oil level. From then on the gauge would bottom out for a few minutes then return to normal. This went on for the next hour about every ten minutes or so until it finially stayed completely bottomed out. the car seemed fine until the end of the trip I heard a loud rattling. Once I cut the car off and let rest it would return to normal and the clattering would stop. did I do damage to my engine? What are the repair cost for something like that?

Asked by for the 2003 Ford Expedition
It could be a deffective oil pressure gauge but any time you hear a noise it is not a good thing. Do you hear an noise on initial start up form cold (rod bearing noise) ?
I would suggest without driving the car, drain the oil and filter, drain the oil into a clean pan and inspect for flaky silvery debris. I would screw into the sender unit position a mechanical oil pressure gauge and verify actual engine oil pressure and see meets Ford specification. Autozone have some popular workshop manuals for free on line once you register (for free) on their website .There is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You get a one year subscription to year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual. or try and get an online repair manual subscription.