Knocking sound

Last Saturday I drove my car all day, had no issues other than the squealing sound when it first started. After coming home had to run back out to the store, again no issues. When I came out of the store to leave, when I cranked it, a knocking sound started. It seemed to stop or quiet down when I would rev the engine and/or romp on the gas to speed up quickly. Got it home and haven't driven it since. My Ex looked at it the other day, and now its having issues starting like the battery is drained/draining. Once he got it cranked, if he pushed the gas all the way in it would stop knocking for a minute. But it knocks consistently from the time you start it till turned off except on acceleration.