Out air conditioning unit is probably 25 years old and cools well. However, in the last few months it makes a booming noise when it shuts off? Any suggestions?

Sounds like some ductwork iis "oil canning ". Have someone stand under the ductwork and listen for the sound. You can usually shift the ductwork a bit to realign the metal.

What is "oil canning" (the ductwork ?

What is "oil canning" (the ductwork ?

You posted a separate question. I answered there.