Ground tests don't mean much for Imac.
You can adjust your throttle travel to get under the db maximum.
That's perfectly legal.
The only thing that counts is the opinion of the judges listening while you are flying.

Working to make your plane quiet for the ground test is a waste of time. If you do a stack ranking of planes for noise with a ground measurement, you will find that the ranking is not the same in the air. Planes that are quiet on the ground can be loud in the air if the prop rips.