What is start up sound?

With that mileage the sound is probably a bearing going out in the starter, they don't last forever, the rattling sound is most likely the timing chain hydraulic adjuster as it is powered by oil pressure, might be time to replace it, and the guides and chain can be checked while the job is being done.

As far as worth it, have you priced another vehicle? You most likely would be money ahead fixing this one if everything else is in good condition.

First, I really like Avalons..... Question, why are you asking people online about the sound your engine is making. We can't hear it. It might be an engine problem. It might be a loose belt. it might be an exhaust sound. Have a good mechanic, and I'd recommend a Toyota dealer mechanic, diagnose this. The repairs might cost a few bucks, or it might be the financial death-knell for the car..... Nobody here can really tell you.

Thanks ,grumpy