Rattle when under a load or going up hill

Could be one of 2 things that come to mind. Firstly get the oil pressure tested with a mechanical gauge, report the findings at cold idle, hot idle and hot @1500RPM. Also you me be hearing what is called detonation or engine knock, to check this out get a scan done and see if a code is set for a faulty knock sensor, as the ignition timing is constantly adjusted by the ECU pending engine conditions, if the knock sensor has failed the timing will not be adjusted and engine ping or knock will be heard on acceleration.