Automatic transmission noises can be minor or serious; it really depends on where it's coming from. Automatic transmission noises are often very different than engine noises. They're usually constant, either a whine or growl during transmission operation. Sometimes you get the occasional tick or thump as well. It's hard to describe, but transmission noises sound very different than engine noises to me.
One thing to pay attention to is when the noise occurs. Does it happen with the engine at idle? When shifting gears? When going from drive to reverse? When cruising at a certain speed? Any information you can gather will help you nail down the source of your transmission noise.
Some automatic transmission noises can be cured with a simple transmission fluid change. If you have a transmission noise, it's worth considering this possibility. A word of caution on changing automatic transmission fluid: It is true that if you haven't changed your automatic transmission fluid in some time, shortly after you change the fluid, the transmission can fail. The reason for this is that the new fluid will flow differently than the old fluid. Old automatic transmission fluid is thicker and full of contaminants. This will actually help seal an older, worn-out transmission. If you remove that old, thick fluid and put new, thinner fluid in, it might leak past the seals that were intact and sealing before you changed the fluid. So if you decide to change your automatic transmission fluid, proceed with caution. However, if you've kept up on your maintenance, you should have no trouble changing your transmission fluid.