2.5L engine noise

I have a 2007 Nissan Altima that has 91,000 miles. About 10,000 miles ago when an oil change was about 500 miles away I would start hearing a rattle in the top end of the engine upon start up. As the weather got colder it started doing it all the time, not just on start up but up to about a minute after, especially on acceleration. I've tried changing oil filters, adding supplements to the engine oil, and changing the oil more frequently. I took it to the dealer after finding two TSB's for those symptoms that recommended flshing the PCM. Their technician in his "infinte wisdom" told me it was the exhaust manifold heat sheild vibrating, after i removed it in the parking lot and asked him to actually diagnose the problem, they couldnt figure anything out other than to tell me they could start replacing parts, which I didnt allow. Any Ideas?