Slap, Tap or knock, what can it be?

[Mr. Spool]

Yesterday I noticed that if I accelerate slowly I get a slight tap slap or knock it is very feint and it only happens between 1900rpms and 2900rpms. We think it is coming from somewhere on top of the engine but I cant figure it out. I figure if it was a rod knock it would be consistent and not come or go. Does any body have any idea what it could be and how I could check different things? thanks.

[Mr. Spool]

Some more info, I went out side this morning to see if I could hear it better, but I didnt make the noise, It appears that if only happens when it is hot.