So I have this old Toro 118 --- 52 inch deck --- 18HP Kohler. It's probably about 10 years old but I had a new engine put in it about a year ago. When I first got it (very used), it worked fine but, when going up hills, it would make a low, moaning sound .... sort of like it was straining. I never gave it much thought because the mower (same type) I ran on golf courses, years ago, did the same thing. A couple of months after I had the new engine put in, I noticed it making the "low moaning" sound all the time. The engine runs fine and the "moaning" only happens when you press the "go" pedal. As soon as I release the pedal, the moaning stops. The moaning sound gets a little louder when going up hills or going up the trailer gate. I checked the hydraulic fluid and it was almost empty so I thought I had found the problem. Filled the fluid but ... no difference. I can't tell if it's a belt or the hydraulic system. I took it to the dealer and he didn't seem too concerned. He thought it was something to do with hydraulics and just took a "they do that when they get old" attitude. Does anyone know what this sound is --- or, more importantly, how to fix it? --- Thanks so much for any input.