Hi,
Excessive fan noise means that either the fan bearing is defective, or that dusti inside fan is making noise.
The best thing to do is replacing the fan. In some case hoovering the fan may fix the problem. Hoovering may damage electronic parts and should be done from distance using a low power hoover.
That is one of the reasons why replacing the fan is a better choice.
The fan is held by four screws, and it is not much different by a computer fan. As a disclaimer I must remind that this is a plasma TV, and that is should be repaired only by an authorized Hitachi tecnician.
Here the spare part for this model.There may be one or two fans (there are variations of same model). There is a fan in the H1 panel, that is usually making the noise. Details and disassembly explained in service manual.
I uploaded service manual here:
Click here to start download..If the link does not work, start from
HITACHI 42PMA400E PLASMA TV SM.PDF.zip