That rattle snake noise is a worn chain and tensioner. And just just like an angry snake,it will bite you!!
You can do it yourself,just do a search here regarding it and buy a chain,tensioner,the top three rails ,three pair of quality vice grips and you are ready to go.
BTW,how do you tune down a car to pass emissions?
it sounds like they leaned out the idle mixture. a sure way to destroy the exhaust valves. Get "them" to retune the engine correctly.