Today i was driving down william st i took my foot of the accelerator and the car slowed down still in 4th gear to abt 50km/h, then i step on the accelerator and there was a loud thump noise. It sounded like it came from under the drivers side floor where the pedals are. Anyone had this problem?
Oh yeh i also had the same noise from the same place when i started my car on a hill type slope(frens driveway)

Can u feel the thumping throught the Gear shifter when your hand is on it by any chance

no cant feeel on the gear shift. It just happened again when i started my car and it was a flat surface

Check the exhaust mounts. I had a new downpipe fitted to mine and it wasn't tightened up properly and it did this.

Driveshafts catching up on eachother?

Mine does this on hard gear changes, supposedly you can get some higher quality bushes and it stops the thumping noise. It may not be this but I would not be surprised.

Anything (one) laying on the road?

Lost your cat? check kKks tyres...

I'm guessing because it happens on startup thats it's not related to tyres or driveshafts as they aren't moving yet.

Check your exhaust, engine and gearbox mounts for play.