i know someone else who's got a 95 with 3.1L & it will do just like you said (only when cold, once warmed up, it's gone) - but here's the thing, it totally dissapears the moment you put it in gear (neutral still has the sound) -- and no, bringing the rpms back up while it's in gear doesn't make the sound return... it stays gone while it's in gear. i've been told it's probably a transmission problem, but since i've not heard it myself, i'd have to be weary & check around some more (does yours stop when you put it in gear?)