Excavator is booming while idling

[MyEquipment]

I have recently bought a used track excavator for sale which is having trouble now. When i idle the machine after a usage of few hours, the excavator's boom suddenly drops downward which i didn't notice at first. The concern is getting worse day by day and puting other's lives at risk.
I have checked personally there is no leakage and wear in hydraulic fluid levels. This might or migh not be the issue. If you have g throught the same proess, you can help me out.


Thanks