engine noises can be diagnosed by there beat, big end knocking at idle sound like a thump every half second or so, whereastop end noises are of higher tone and at a higher tempo. However big end knocks rarely dissappear when engine warms, so id rule that out. It could be a little piston slap which would indicate maybe slightly worn bores, but I doubt it. I suspect all you really have, is that older valver favourite, sticky tappets on cold start up, dont fret not serious, more annoying. Reason I dont think your engine bores are worn, is I have just had head of mine, which is also on 90k, and I was absolutely amazed to see the original honing marks in the bore, absolutely no wear whatsoever, good bottom ends, oil spray bar under pistons really done its job. Try an oil change and flush, see if that shuts it up, but really doubt thats its anything worse than this mate, thats good news