GQ Patrol banging noise

Howdy People,

Can anyone help me with finding the source of this banging noise. It doesnt happen all the time, just every so often...the vehicle makes this banging noise that sound like it is coming from under the drivers seat or in the middle between the front seats. Its been up on a hoist, we hav prodded and poked every thing that we can find under there....and still have no idea what causes it.
When its raining and the roads are wet...it seems to get worse....but like I say....the banging noise isnt there all the time..I can go ages without hearing it...then all of a sudden it will bang away...then it will bugger off again.

I get some strange creaking and squeaking noises out from under her at different times...mostly when I have been down dirt roads. If I hose her off underneath...squeaking goes away...I thpought it might be rubbers or something like that getting dust between them.

cheers
Tazz