I have a 1998 Tahoe, 294,000 miles. Recently it developed an intermittent problem of a momentary rattle noise (louder than tappet sounding) at startup when the engine is hot/warm. Never an issue at cold startup. Then today after warm startup, it had a rough idle (whole vehicle shaking) but it went away after driving and cannot recreate the problem. No DTCs. Any idea on where to start looking? Thank you.