Magnum engine's got a tapping sound....

I listened to the audio clip and it sounds like valve train noise alright. I'd pull the valve covers and see if any lifters are collapsed. You'll be able to tell that easily if there's any pushrods real loose, i.e. no preload. If any crud got into the system and went into a lifter it can clog it up and cause it to not pump up.

BTW: Your positive the new pushrods are the same length as the old ones, right?