While driving my truck I noticed a louded knocking or clicking sound, also whirring. Drove for a ways and the oil pressure dropped, I had just changed the oil a few days before and checked it just minutes before. I checked oil again and looked to make sure oil filter was placed right. Everything was full and the truck was not showing hot. Parked it at the house and the next day I drove it around (ran great and smooth for about an hour and then it started it again. What could cause this. Oh yes when I checked the oil the top of the dipstick was hot but the bottom and oil were cold.