nate.dawg

I have a manual 08 350z with 56000 miles on it. I had a mean exhaust setup with aam test pipes/aam y pipe/agency power single. After about 1500 miles after that install the I would hear this faint rattle sound that seemed to be coming from the engine bay only when the rpms were between 2-3k rpms. The rattle sound was loudest when engine braking in a low gear between those rpms. I tried pushing in the clutch to see if the sound would go away but it was still there between 2-3k rpms. I took to a local mechanic and he said it's the lifters that are going bad and if I took it to nissan they would have me get a new engine. I freaked out and decided to get an aftermarket warranty with endurance so they could pay for my new engine when the time comes. Now, to minimize my risk of a denied claim, I put my exhaust back to stock. What's crazy is that when I drove the car with the stock exhaust, the sound was completely gone. My head is spinning because now I'm thinking it had something to do with the exhaust. If it was a lifter problem would it be louder or quieter depending on the exhaust? What do you guys think? Do I pay $145 to get it diagnosed by nissan or do you guys know what it is? If it is the lifter do you think endurance will pay for a brand new engine? Thanks guys.