If anyone has any idea what this is, or what to do, please advise:

There is a pretty loud buzzing coming from the ceiling just as you walk in our backdoor (in a small hallway). I can also hear it in one area of our kitchen (internal wall). My husband went up in the attic, and said he heard nothing. We have a one-story home, built in 2001. It happened once before, and just went away. I can't really remember what happened...I just know we heard it, then must have left the house to do something, and I really didn't think about it again until tonight. I tried turning off both AC units (we live in Houston and it's getting humid, so our AC is on). That did nothing. I also thought it may be a light bulb ready to go out, but it is not. I can "hear" the vibration when I put my ear against the wall, but just have no idea what it could be. My hubby is going to shut off the breaker and see if the noise stops...we don't even know if it is electrical or not. Does anyone know who to call for an evaluation?? Do I call an electrician?? Any ideas would be helpful, and thanks in advance!

kathy