Kobelco hydraulic issue

Hey guys, I recently picked up a well used Kobelco sk70 sr from a local construction company. To say the least, at least in this case, maintenance on this machine has not been a top priority. I purchased it knowing it would need some tlc. Anyhow.. the issue I am having is when the machine is parked but running, it sounds like the hydraulics are "working" . It sounds like there is a load of some sort on the hydraulic system without touching anything. But, as soon as you pull a function, say to swing the machine, the loading up sound goes away and it operates fine. Its kind of like hydraulic fluid is deadheading somewhere but as soon as you move a joystick and give the hydraulic fluid a place to go, it takes the load off of the machine and it sounds normal again. The one thing that comes to mind that it sounds like is when for instance you turn on your auxiliary hydraulics to run an attachment... with no attachment. The engine loads up trying to push fluid but has no where to go. It sounds like that when its just parked. Some key things... overall the hydraulics seem pretty weak on the machine. It digs fair but seems very underpowered. The travel motors and swing motor seem very very slugish. Also the hydraulic fluid is milky. Looks to have a lot of water in the system. I am waiting on hydraulic filters to get here and plan to drain the fluid and replace with fresh as a first step. But im no hydraulic system junkie! Any ideas? Thanks!!