Ticking noise coming from engine?

Ever since I replaced the spark plugs in my 01 Ford Sport Trac, I've been hearing a clicking noise coming from the engine on the driver's side. One plug I replaced had been cross threaded, so I had to buy a tap to repair the threads in the block in order to get the new plug in. They were Bosch plugs. I read later that Bosch typically doesn't work too well in Ford engines, so I switched 3 of them out with Motorcraft. Still, the noise persists. It starts in idle, but gets louder and more frequent with increase in RPM's, and quiets down when set at a steady speed and decelerating. I have pulled each plug wire off one by one to see if they are the cause, and I still hear it. I feel like it has something to do with the plugs because it started after the initial replacement. Any suggestions on what this could be, or how serious it is? It's very annoying. The vehicle has 203k miles for additional information.

Check the plugs again, make sure they are in tight. Motorcraft is best in Ford engines, so replace the rest of the Bosch.

Not likely but the reach might be too long. But I think if you were hitting the plug you would know it. If a plug boot is not fully seated it can make a ticking sound. Watch the engine in the dark. You may see a fireworks display.
This might be lifter noise or even rod knock, broken ring etc etc. Most likely something in the valve train.

Ticking noise is quite horrible in our vehicles, mostly with old engine we have found these kinds of problems. But with new engine whenever we have found anything serious or ticking; we should understand that there's some trouble. So it is quite better to visit a professional mechanic to deal with these kinds of problems.