What you are describing sounds like pre ignition caused by the carbon build up getting so hot in the combustion chamber that IT will ignite the fuel/air mixture before the piston has had a chance to compress it all the way and basically it is beating the hell outta your pistons (as the piston is compressing the fuel/air mixture, the heat of the carbon buildup ignites the mixture causing your piston to try and compress against the premature explosion that is supposed to force it back down). I would get it looked at right away even if it means having to pay the 180 cause it WILL damage your engine if you keep driving it like that.