are you , by chance using a loc, or high level input? sounds just like a mid is working like a mic and feeding the signal back to the amp is the sound present with the rcas unplugged? also, the noise by tapping the amp does not always mean that it is bad. your body does pick up charge, and touching can have an effect if you are experiencing an rca-level problem in the vehicle. what does your meter zero out at? you have to subtract that from the reading you are getting, so it might not be as high as .5 ohms, really.