I have a 98 Expedition 5.4 with a recent 100k replacement used engine put in. Oil was changed about 3K miles ago with Mobil Synthetic extended performance 5w30. I have noticed on my last 2 trips towing my boat that after the engine got warmed up about 8-10 miles into the trip, (WITHOUT OVERDRIVE ON) running at about 2500 RPM'S there was a pretty substantial rapping noise coming from engine. Did not sound so much like a bearing knock, as much as a lifter rapping noise. After the 1st time I heard it, I noticed I was about a quart low and topped it off, but 2nd trip was the same. On my 3rd trip running at about 1500-1800 RPM'S, I did use the overdrive and there was no noise to be heard. Should I change the oil type, weight, or add an additive? Do you think it is an engine issue or more of an oil breakdown issue.