I was given this unit by a friend and now when you turn it on there is a sound that's coming from the back, sounds like a hard drive booting up, it never goes away until you turn unit off
How familiar are you with DLP TV's ? Because this is most likely your color wheel, and this can be a normal noise, even more so if it is an older unit.
If the sound is erratic and/or very loud then I would be willing to bet that your color wheel is going out.
Another possible culprit may be your cooling fan getting old and starting to get loud..?
If the sound starts as a whirring noise from the TV nearly as soon as you start it up then its most likely just your color wheel.