Powermac G4 stuck on grey screen with loud fan nosie

Hello,

I wonder if anyone can help me.

I have an old Powermac G4.

On starting up, the chime sounds and the screen turns to grey and shortly after the apple symbol appears as per normal.

However, it then remains stuck on this screen and shortly after the fans (I assume its the fans) start making a very loud noise.

I purchased this computer second hand, from a company who were about to throw it away because of this problem.

At the time I called an apple engineer who came round and fixed the problem, as I remember fairly quickly.

The computer then worked fine for around 3 years.

I moved home recenly and transported the computer (safely and securely) in the back of the car. However, on set up the problem had re-occured.

There are no start-up discs as the computer was destined for the rubbish bin, and if I can avoid calling out an engineer as I did previously then that would be great!

If anyone has any ideas on what I could attempt then I would be grateful.

Thanks in advance

Jamie D