A weird sound(knock) when accelerating

Car: 2001 Volkswagen Passat V6, 30V - Automatic (100K)

With in the last week when I go to accelerate from sitting still I hear this knock as I hit the gas. It's almost immediately after I push on the gas. If I push on the pedal lightly from the start it won't happen so I don't know if a problem is in it's beginning stages but just a normal push on the gas to keep up with traffic and "Knock!".. I am scared to think that it is something with the transmission....It's an automatic.
It doesn't happen after that.. I could be going 30 and quickly accelerate to 65 and everything is normal..

sigh........
could it be a joint or something..... what's it called... The pumkin???? something in there? The car has just under 100K now and I don't think anything major has ever been replaced. A typical electron part here and there. I don't see anything common amongst Passats in and around forums (except for some mentionings about oil sludge)

Im open for anything suggestions as you can see.. Confused

I have a 2000 Passat 4motion V6 Automatic 124,000 KM (about 77,000 miles).

Same Issue.

This is the second time with this same issue. Last time I was told by a Mechanic that this was predetonation or Pre ignition spark. This is usually caused by low octane fuel. I changed the grade of my fuel to high test, a couple of tanks later she stopped knocking. The mechanic also told me it could have to do with worn plugs. These were the cheaper of the solution that he wanted me to try first to reslove the issue.