I can assure you it's not a bad lifter. Our engines just don't have 'em
The cam directy interacts with the valves so there is no lifter/pushrod combo in the mix. What you're describing sounds like it might be in that neighborhood though. Given your description I would tend to suspect a stuck valve or related component. Perhaps the valve is returning to the closed position when the piston is on it's compression stroke and the spring hangs up for a second then slaps up against the assembly. With your bad oil pressure you might have gotten a little dry on top and done some damage to the head. Get a second opinion before you get to worried though. It's hard to diagnose from here.