Well, with hydraulics it's usually a set-em-and-forget-em deal but it sounds like you have a problem with leakdown. They shouldn't leak down overnight and take 10 minutes to quiet down. I've got hydraulics on mine and I can leave it for a week and they won't leak down much and any noise disappears as soon as the oil pressure comes up. Could be your oil is diluted with something, gas or water. Can you see or smell anything in the oil?
Has it been awhile since the last oil change? If it's been awhile (say 6 months or more) I would try that first using a good quality oil.
Good luck, hope it's nothing too serious.
