I recently purchased a 96 cad sts and it has 200,000 miles otherwise perfect cond. Prior owner used premium gas which is required. I was cheap started putting cheap gas in and I just started hearing a rattling noise at about 2800 rpms. Also when I first start it I hear a rattle and little puff smoke came out exaust? Seems to be getting worse anyone know what it could be?

Your valve seal may be going. I would go to a auto store and ask for octane boost it only cost about $3.00