weird knocking noise when running in neutral with clutch out

[ryanruggles at uniserve.com (Ryan Ruggles)]

Hey listers, I have a question.  When I start my car in the morning to warm
it up, in neutral w/ clutch disengaged, I hear a fairly large knocking sound
coming from under the hood and it sounds like it's coming from the tranny.
When I press the clutch down it stops but as soon as I disengage it, the
sounds comes back?  What do you guys think it is?

Ryan