When the a/c comes on the return duct drop at filter buckles in and u can hear the blower

Has it always done this or is it a new occurrence? Are any of your return grills covered by furniture or anything else. The return sucks in air, if it can't get enough air it will collapse the duct work as you described. Either you have some covered returns, or your duckwork is undersized. Depending on how close to the furnace your return grills are, it is quite common to hear noise from furnace blower(or just air flow). Are return ducts insolated. Lots of factors to consider here. Please let me know more. Lee.

Sorry, you didn`t say so I have to ask. Is the filter clean and no possibility that filter is stopped up