Strange knocking sound when my '98 V6 is in Drive (or 1 or 2)
 

I only noticed this for the first time yesterday and I'm pretty sure it has not happened before then.
I had been out in my car and when I pulled into my driveway and before I had even put my car in park I could hear this weird knocking sound. I cannot tell where it was coming from. So I took my car out for a drive around the neighbourhood and could not hear the noise. I pulled over and left my car in drive and the noise started again after a few seconds of idling. After that I could hear it as I was driving for about a minute and then it stopped. It sounded louder as I went around a corner.
So my car only seems to do this occasionally
ONLY in Drive, 1 & 2 - not in reverse, neutral or park.
I bought it used 3 years ago and do not drive it hard. oil was changed last month and is at a normal level. Transmission fluid has been changed less than a year ago.
I'm unsure whether the road conditions may have something to do with it as the roads were heavily slushy yesterday.
I called my mechanic about this but of course the car was making no strange noises whe he was on the phone!
I'm really hoping somebody can help - you can also e-mail me at njh_04@hotmail.com - please note I am not technical.
Thanks in advance, NjH

do you have an idea where its coming from? like the engine bay or suspension? and basically it's only in your forward gears?

It almost sounds like it is coming from the steering column or directly underneath that like maybe an exhaust pipe. If not that, then it must be the engine bay. And yes, it is only in forward gears and it does not do it all the time.
Thanks :)

i had that problem on my old chev truck. for me it was the flywheel starting to come loose. but it could also be exhaust hitting somewhere

I had a similar problem with my F150, it was the exhaust banging on the rear differential on even the slightest bumps. The moron who put it on didn't hang it with enough clearance or proper hangers and it was bouncing off my rear diff and crushing my brake line. But you say its more towards the front. Could be loose exhaust still I suppose.