2004 hyundai sonata knocking on passenger side of engine

When I start my car, and it is idling, it sounds fine.
When you put it in drive, it begins clattering. At
speed, no noise except when I hit the accelerator
and take my foot off the accelerator quickly. Once
the engine is fully warmed up, it is not as bad.
When I pull up to a stoplight, it clatters, but if I put
it in neutral,  the clatter stops.  It is noisier in cold
weather, not as bad when the weather is warm, but
that could be coincidental.  Any ideas of what it
could be?