On other Bobcats I have had, over the years, every time I heard this insane growling noise, it was always worn out hydro pumps. But in this case, there is NO lack of power in any direction - in fact has more drive power than I ever expected a 642B would have. Changed hydraulic filter in rear (Bobcat filter), drained all of the hydraulic fluid I could and replaced with the Bobcat fluid. No change. In fact, it seems to be getting worse. But absolutely no drop in performance. Loaded or unloaded - makes no difference. If you bump the throttle all the way up, it will reduce the noise....some, but not much. If you shove the steering levers all the way forward or backward, it will quite down to a reasonable level, but at low engine speeds or slight pressure on the steering levers, it is virtually unbearable and makes you think it is doing some damage to the pumps. I checked for air intrusion, collapsed suction line, but in my mind, either one of those would cause a severe drop in performance. Not in this case. Forgot to add - drive pressure at idle is more than sufficient to turn the machine left or right and forward/backward. If this was a case of worn hydrostatic pump, I should think that it would not move at a low idle, but it will. In fact, you have to throttle it down to turn on pavement as it will get to bucking uncontrollably because of high drive pressure. (Yes, I completely replaced all of the steering lever linkage and adjusted same - no creep). Could this be related to the relief valves? Hoping those with more experience/knowledge than I have, can shed some light on this?
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