ralph replied:

Try the chassis supports near or under front radiator these compress over time and need to be replaced.You can tell because the metal crush tube is sticking out past the rubber mount.

Stevo replied:

Les, I agree, the particular squeak you describe used to drive me absolutely insane. I fixed it by selling the vehicle !!
It is emanating from the hinge guides within the firewall. A temporary fix is to use a can of CRC with the little directional hose attached and blindly squirt it into holes where the hinges enter the firewall. the bonnet must be as low as possible when doing this. Good luck.

Cobra replied:

Les, try a can of Tri-Flow silicon. I use it on squeeks, roller doors, lock tongues etc and seems to work ok. You can get it at locksmiths, about $15

les replied:

Problem Fixed !
After numerous efforts I finally managed to track down and fix this problem. It was coming from the unsupported areas in the centre of the bonnet. I bought a 4mm thick strip of hard rubber, and glued this along the inner lip of the scuttle panel, beneath the windscreen. (the lip where the inside edge of the bonnet rests when it's closed, the side nearest the windscreen) This is invisible when the bonnet is closed, and the result - Silence !
Just in case any other GQ owners have this problem......

Cheers, Les