There are several different noises which can be emitted from inside the case. Some noises are normal and some can be indicative of a problem.
It is common for the internal fans to emit more noise when you first turn on the system, they will quiet down when they reach full speed and balance out. Dust build-up will accentuate this behavior.
Fans will accumulate dust under normal usage. Dust build-up can cause the fans to operate much louder than normal and cause grinding or loud whirring noises. Cleaning the dust build-up with a can of compressed air can help.
You may notice a whirring noise when inserting a disc into the CDROM or DVDROM drive. (This can also occur when accessing the CDROM or DVDROM drive after a period of inactivity.) This symptom usually occurs only when a disc is spinning up to reach normal speed and will quiet down when the drive reaches full speed and the disc balances. You may find that only certain CD or DVD discs create noises. This is caused by an unbalanced disc and is not the fault of the drive.
If you have questions about any noises, please call technical support. A technician will be able to listen and help determine if it's normal.