Are you playing Amateur level? If so, check your task list. The game isn't particularly linear, so depending on what you've done the buzzing noise might be due to appear later on. Just continue playing and see what else can be done. If you're playing by the walkthrough, check back closely against it to be sure that you've picked up/read any documents, spoken to everyone about everything, etc. It's often just one small thing overlooked that holds up gameplay.
"Bleat, Watson -- unmitigated bleat!" ~ Sherlock Holmes