Hi I own a GV diesel 08 and I'm experiencing a noise from the front suspension... I think it's the down travel knocking.
It seems when I go over like a speed bump, the wheels shoot down and make a loud bang.

Anyone encounter this before?

I don't have that model, but I have experienced it - the aftermarket struts were the cause.

Really?? How did you over come it?

Replace the struts...

Were the aftermarket struts crappy ones off ebay? I bought some cheapo ones for a corolla and it use an utter failure. Lasted a few days..seriously.

That's a bugger... Thanks. What did you go with in the end?
I have quickly priced up ironman at $185 each.

Have you checked the upper strut mounts?

Have you checked the upper strut mounts?

X2 on checking the strut mounts.  If they are aged/worn the rubber is probably torn and not securing the top of the strut, which allows it to bang on the strut tower/s during bumps in the road.  The top of the strut should be centered within the strut mount.  If you have a bad strut mount the top of the strut will be off-centered like in the pic below.  You can actually see the torn rubber in the pic. 

Thanks guys this is what mine looks like...

The bolt looks off-center in the pic.  Does the other strut look off-center too?  They should be centered inside the body opening and evenly spaced between the three mounting bolts.  Doesn't appear to be in the pic.

Sorry I have been having server issues and it would let let me post... It's just the picture angle they are centred.

I think I'll have to have a go at disassembling the strut and looking at it. But I'll put up with it until breaks then replace it.
It shouldn't be unsafe though right?

The strut is what is keeping your wheel upright.  When cornering, the side pressures increase and having the strut wobbling shouldn't be a big deal, unless it folds over and no longer controls the turn.

I would take the nut off the top of the strut, then jack it up at the A-arm, put a jack stand under frame, remove wheel, and lower the A-arm.  When the strut comes free at the top, check it for resistance (it should be pressured up to the top), if no resistance it is shot.
You should check that your bump stops are long enough to keep the struts from bottoming out.  Bottoming out, or topping out your struts are the quickest way of destroying them.

Ok thanks I'll look at that on my next days off. I have a trip to the Flinders in 2 weeks. Perhaps I'll look at it afterwards as I have just had it aligned a couple months ago.

I would take the nut off the top of the strut, then jack it up at the A-arm, put a jack stand under frame, remove wheel, and lower the A-arm.  When the strut comes free at the top, check it for resistance (it should be pressured up to the top), if no resistance it is shot.

A word of caution - I would not suggest this course of action on this model of Grand Vitara.

Unlike the previous generation, an 08 GV uses macpherson struts with the coil over the strut, the coil is retained by that nut, if you want to disassemble the strut to check it, remove the assembled strut from the vehicle and then disassemble it with the appropriate coil spring compressor, the risk of damage to the strut top components if done "in situ" is very high.

I did not catch that this was a third Generation vehicle.  I know that 1st & 2nd Generation were the same.
Fordem, Thanks for the correction.