Sudden momentary dips in aws rds free storage space available

I have been experiencing some sudden momentary dips in the available aws rds free storage space available. Once the free space dips it goes into the storage optimization mode. I have a postgres table which is around 100gb with around 280 mill records to which constant updates happen. The table has a toast of around 300gb. I'm not able to think where to even point the issue at while looking at cloud watch. These dips are making the product a lot slower. I have looked at enhanced monitoring and seen all the different charts and can't see any unusual spikes either with os or db processes in cloudwatch during the time these dips happened. Below is an attached picture. Please let me know if you guys have any ideas why this might be happening?