Q&A

 Master

If the old oil was thickened from overuse, the tapping noise could have been damped some by the oil. Fresh oil would be thinner and might allow more noise to be heard. If the 'slight tapping' is in the top of the engine, it's probably valve noise. If it's lower, it's probably a piston noise. If it were a harder knocking, it would be a crankshaft ('lower end') noise. Slight valve noise is usually not much concern, but the lower in the engine and louder it is, the more concern. You may want to bring it to a GM specialist for a second opinion.
If you want to find a specialist, here's a directory link for you:
https://repairpal.com/directory?address=20748&car_brand_n...

 Master

Did you just buy this car? Have you owned it since it was new? Who has been changing the oil for you? What brand/weight of oil have they been putting in your car? How often do you perform oil changes? How many miles are on the engine?
These are some question we need to know to help diagnose possible issues related to your question.