Slight knock after car sat for a week

[chomptown]

1983 240D

The car sat for just about a week, and when I started it up this morning it had a slight knocking sound. Hard to tell if it is coming from the injectors, but that was my first idea. I checked the oil and topped it off just to be sure. When I put it in Reverse the knocking went away. Put it back into Park and it was still there.

Something I've noticed since I changed the filters out last, is that the inline filter always has a slight amount of air in it. The hose that feeds into that filter is a little old, and I've always meant to change it out. Do you think it could be drawing air through that hose and causing this knocking sound? I had my wife drive the car to work, and she said it wasn't knocking after it warmed up. Would appreciate some ideas from you all.