I have a 2001 exployer sport trac 2wd. After truck has sat overnight or several hours i have a noise from top of engine, sounds like lifters (engine sounds exactly like it has no oil pressure). Oil pressure has taken up to 15 to 30 seconds before getting reading a couple of times. I have only had truck for 3 months, eng was doing this when i purchased i didnt notice because eng was at operating temp.