Air is about 1/250th as dense as the hydro fluid, it can go places the fluid can't. Kinda like gortex raingear, water dosn't pass but it still breathes. A leak on the suction side piping would draw air and not leak fluid.
A low resovoir could also be a problem. An extended cylinder uses a little more fluid from the tank than a retracted one due to the rod taking up space in one side. Depending on how many cylinders you have extended you may be drawing the level low enough to suck air. The pump does not pass air well and when air hits it, the system pressure falls fast, then when the fluid hits, the pressure spikes. This could lead to jerking/erratic operation. I would check the suction pipeing, fluid level and the suction filter first.
As Mark M mentioned this air passing through the pump would make unusual noises even when you were not working the controls. With no controls being moved, if the pump has a steady supply of fluid, it should make a steady sound. You can listen to it really well in operation with a mechanics stethescope(handy tool, only a few $ at Harborfreight.com tools), or a long screwdriver/rod, one end held against the pump body and the other held against the bone behing the ear(with ears plugged). This blocks out the other engine and gear noises. With a valve cycled, you should also hear a "woosh" sound as fluid passes through the valve going to/from the selected cylinder. If the cylinder reaches the end of it's stroke, you should sense a momentary drop in engine RPM as the pressure and pump load spikes and the relief valve opens. If the above didn't help, then I would look into a pressure gauge installation to monitor the pressure and relief valve openings.
Good Luck.
