ever since i bought my car I has not accelerated smoothly. If I'm just driving normaly and then I step on it i sometimes kicks or bogs before giving some power and sometimes there isn't alot of power. i also noticed that you can sometimes hear some kind of ticking/crackling sound from the engine in the 6,000-7000 rpms when the window is open (1st and 2nd gears). Could this be the sparkplugs. it has about 40,000 miles on it. Also the guy I bought it from blew a piston in it but had it rebuilt at the subaru dealer, if that helps. Can some one PLEASE HELP ME? THANKS!!!