Interesting Transmission Problem?, Can anyone give me some insight??? Options

[CanadaCelica]

Hey everyone. I have quite the interesting symptom going on with my transmission. I hope maybe someone could help me out here with a diagnosis.

Hers the situation,

When ever I make right turns or am turning the wheel more to the right, say accelerating onto the on ramp for the expressway I hear a strange thudding noise. Only thing I can think of comparing it to would be tapping on a wooden table. It only happens when I am in 1st gear and is the most noticeable in second gear and at a slower speed. Once I am in third and going about 50+km it can’t be heard.
This noise also never happens if I a have the clutch pushed in or I am in neutral coasting. And it can really only be heard inside the cabin and seems to be coming from around the center of the dash but right by the floor.

The problem first showed after I got my clutch changed but didn’t start till about 4 months after the change.
It doesn’t affect performance at all or I don’t notice it affecting.
Another thing, I don’t know if this would have anything to do with it, but on the passenger side axel where it goes into the transmission there is a small leak. I believe the seal has gone.

Well that’s all I can think of for now as symptoms, if anyone has a clue as to what this could be that would be great.