I have a ZD28 that was purchased new and only used residentially. The unit has approximately 390 hrs total. It has developed a noise in the midsection that appears to be hydraulic of sorts. The hydraulic deck will not raise and lower. The noise will occurr shortly after startup and the mower will lose its ability to travel. I'm hoping someone will tell me that I could change the hydraulic filter and fluid and help but im in the dark. I am mechanically inclined and have done all sorts of mechanic work. I am not afraid to tear into it but would like a little helpful insight. The fluid levels all appear fine and the fluid appears clean. I have not changed the filter since the problem arose. If someone who is knowledgeable of this problem or symptoms and can help a feller out it would be greatly appreciated. Thanks in advance.