hmmm...i suspect your hydraulic line is partly clogged up resulting in a slight increased in fluid pressure but not high enough to activate the pressure relief valve....your observation that this problem disappears when the pump is idling (ie. under no-load conditions) confirms my hunch...
nonetheless, in order to confirm this, you have to get a mechanic to inspect whether the operating pressures are within specified values and if its exceeds the set relief pressure, then i guess they will know what to do next...good luck..
