Lately, my macbook has been freezing up very often while not connected to the power. It's 15" early 2011 model macbook pro. When it freezes, it usually makes beeping noise, but sometimes doesn't even make any noise and just freezes. I usually force it to shut down by pressing on the power button. It's always when the machine is not connected to the power, and the bottom of the macbook is quite warm. Is it to do with the ventilation?