To be honest mate it could be anything from suspension, to diff. Can you be any nore specific? I know it's not easy but the more infor we have, the better chance we have of helping. 

Hi, thanks for the replys, happens every morning going up mile long steep hill, the car just starts a knocking noise, it does sound like the exhaust knocking against something, but it isnt. then when you get back on straight road and the car isnt under pressure it goes, seems to be that what ever it is has cooled down. Gave the car some stick tonight on way home doing about 90 the knocking came back, slowed it down for a couple of mins and it went, im at a loss really??

It could be what I've experienced recently. By back box blew up quite dramatically the other week due to the innerds being rotten & collapsing. The tone of the engine changed dramatically under load as the exhaust was partially blocked. Only when under hard acceleration did it sound really weird, like a groaning noise. Then it went with a bang. I'd check inside the rear tailpipe just incase, for what looks like long hairs coming out of the exhaust. The outside of the backbox seemed pretty ok and there wasn't much corrosion to be honest. But the innerds were completely goosed.

Just one possibility