I want to buy my friends 95 civic. He says the 1.5 bottom was rebuilt and he put a 1.6 vtec head on it. I have been driving it around for about a week and at about 1500 rpm and 3000 rpm the engine makes a really weird rattle. I cant tell if it is from the engine or exhaust or somewhere else. He says its from lack of octaine and to run premium through it but I have filled it up with premium and put octaine booster in it & it still rattles. Any advice??