Car making sound sometimes when engine comes on

Hello,

I have a 08 Prius. Over the last few weeks I've noticed that when I accelerate until I get to about 20mph or the engine comes on to charge the battery I hear a strange kind of drudging or scrapping sound coming from the rear area of my car. it only happens when my trunk has groceries and I have a passenger in the back seat so I wonder if it's related to some type of weight issue? I also recently had to replace my stolen cat converter. Any help is really appreciated

Thank You,

 

I'm at around 121,000 right now and as I investigate more I have a feeling it has something to do with the after market parts for a cat converter provided by my insurance.