Tapping Sound? Lifter?

Hello. My car recently starting making a clear and loud tapping sound. The tapping increases in speed as I accelerate. My car is a 1999 Olds Intrigue with the 3.5L Twin Cam V6 engine. It has 109000 miles on it and used mainly on the freeway. Since I have owned the car I have done regular oil changes and maintenance. I switched to synthetic a little over a year ago. Recently, I took it in for an oil change and I had noticed a loss in power and the check engine light. I had the codes checked and there was a misfire. My wife had a full maintenance service done: oil change, transmission flush, coolant flush, spark plug change, and air and fuel filter replacement. The light has not come back on since then and the car is driving well. However, the tapping sound started the day after the services. We believe it's a lifter. We have read that use of transmission fluid or Risilone in the oil and changing the oil and filter a 3-5 times within 1000 miles may clear the problem. Can someone advise or help? Thank you!