Rattling Noise When Accelerating

My 98 Altima makes an increasingly loud rattling noise when it is cold, and only upon accelerating in gear (won't do in park while revving engine). When you accelerate, it sounds like a diesel engine until the engine is warm. When the car is driven a short distance and restarted, the idle revs really high, kicks down, revs high again and eventually finds a normal speed. Not sure if the two are related. Nissan dealer who heard the noise last year told me it is "normal" even though it never made that noise when new. 3 years ago they replaced the idle pulley and that seemed to help, they say that is not the problem. I am concerned that this indicates something about to break on me. Any ideas?