I had an NPI block from a Crown Vic that was in mine when I got it. Just did a PI swap with heads from an '04. I noticed last week that I had a slight tick (not a knock) coming from underneath the car, noticeable exiting / entering the driver's side. I assumed it might be exhaust since I can't hear it in the engine bay. I also have a theory that, because it still had regular gas that had been sitting for 10 months before I did the swap, and I only put enough premium in it to get the bugs from the swap worked out that it is a slight ping. I'm filling it up this Thursday with premium, so I'll repost then.