Well this can go one of two ways. House builders expect creaking sounds on a new house the first year or two as everything settles in. If the tank's stand is new and built having the screws hold the weight it could be advised as being "ok" but I would investigate it thoroughly. Creaking sounds may imply something is going to give. There is a big difference here though, houses have a small distance where it "gives" due to natures forces such as winds and soil erosion that cause the creaking sounds. Generally tank stands are built with no give what so ever having support beams hold the weight.
If the tank isnt leveled properly then that may be a reason but it could possibly be a stand that was incorrectly built having screws hold the weight rather than support beams. A single support beam of a 2x4 can hold roughly 800 pounds where a single screw can hold but a small fraction of that. I would first look at the construction of the stand. How large is this tank?