It could be many things such as...pressure valve or the motor or something in the motor. Your best bet would be to take it in to a service center and have them look over it for you. A lot of places will give you a free estimate before repairing so you can determine if it is worth it.
Good luck!
Don- Toolsandmachinery.com
December 2008
When was the last time you checked,or changed the pupm oil?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix d.i.y tools.