Yes, you can shim up a tank to make it level. Your hardware store will sell the wooden shims that you can put under the stand, where needed. Use a level, and if you don't have one, you can use the water in the tank as a level. If you get is so that it is a uniform distance below the top of the tank, its essentially the same thing as a level.
Using shims to make a tank level is actually recommended, it keeps the pressures and forces created by all that water equal throughout the tank.
Make sure you don't have any bends, kinks, or "u's" in the drain lines. In a typical sump set up, the outflow hoses are larger than the inflow hoses, and have a higher capacity than the inflow hoses. This prevents flooding. The drain hoses are normally not completely filled with water, but have air in them and the water travelling down them. This is where much of the gurgling sound comes from. If you have a bend in the outflow hose, water will stall there, until enough water accumulates to occlude the hose then air and water behind it will force it through the bend. This will cause a surge of water and air distal to the bend.
Besides leveling the tank, you can adjust the depth of each overflow box to adjust the amount of water that goes through each box, and adjust the level of water in the tank. That is assuming you have a tank with hang on the back overflow boxes, I don't know if tanks with the built in overflows are adjustable.
