Ive been driving my 03' Z for roughly for 4 years, as long as I can remember it's made a knock when I start it. I know I didn't buy it like that. But I'm not sure when it started. I'm pretty sure for a couple months I'd been putting in 87 regular gas, then I smartened up and used 91. Everytime I basically come to the car and start it, it knocks for about 1-2 seconds then runs normally. I haven't noticed a power loss. It's an automatic as well, yeah yeah.. The engine has a quiet knock when revved higher 2-5k rpm. I'm thinking timing, fuel tampeners, injectors..?