Question Details:

When you press the ON switch you usually get a liitle noise like a hoover for a few seconds. However this noise is coming on and off at various times.

Answer:

It seems that your computer fans are dirty and need to be cleaned. In most cases your power supply fan and processor fan are making noise when they get dirty.

Turn the power off, unmount your power supply from your computer and take its case off using a screwdriver.

Suck most of the dust out with a vacuum cleaner, then get a small paint brush or old tooth brush to clean the inside fan.

You could use a cotton swab to clean the fan blades with some Isopropyl Alcohol on it.

You can use the vaccuum cleaner, paint brush and the cotton swab to clean your processor fan, too.