Crazy noise plus sudden drop in mpg's

You probably want to check under the car and see if you are missing an under-engine cover. This may explain the noise but not the drop in mpg.

 

I saw my mileage go all over the place before I hit 10k in milage. At 23k now (in 9 months) and I'm getting 50mpg, and no less that 47. I am in ECO mode. Your best bet is judging from gas in vs mileage. It also sound like you oil change door may have fallen of, check into the dealer.

 

As for the noise, I once had something similar that wouldn't go away until a friend gave the end of my exhaust pipe a real hard shake and it hasn't rattled since. Meaning the exhaust system has some heat shielding that can make that kinda noise.
As for the MPG:

I'd avoid Toyota dealers when it comes to addressing these kind of fluctuations in MPG, it's not what they do. In their eyes your car has no problem.

That being said, your decline in MPG could be any number of things, or a combination of things... Your decline in mileage is consistent with the change in seasons, maybe a wee bit more severe...

The bright side is you now have a great opportunity to spend time checking everything out and learning more about your car: Tire pressure? Reboot? (disconnect 12v battery, wait, then reconnect), air filter? too much oil added? heavy reliance on heater and defrosting mode? dirty air flow sensor? drivetrain/alignment changes? State of charge of both batteries?

 

I can't speak to the noise but in the absence of anything wrong, see Why mileage gets worse in winter | PriusChat and Car and Driver: Mileage? No, it's Your Gallonage that Really Counts | PriusChat.

 

Excellent insight thanks... cwerdna

Here's the quote I like the most:

"here's a sneaky illusion in mpg numbers. Consider: If your pickup rated at 10 mpg gets only 9, you shrug and say it's off only 1 mpg. But if you drive a hybrid labeled 50 mpg and it drops the same 10 percent to 45, you complain of lousy mileage. The illusion tricks you once again when you think of mpg instead of the fuel you actually burn. Hybrids are chosen by people who think saving gas is right up there on the list of American virtues with motherhood and voting. But when the hybrid gets 45 mpg instead of the expected 50, a 100-mile trip consumes less than a quarter of a gallon more than expected. Compare that with the pickup that gets 9 mpg instead of 10; its 100-mile trip burns 1.1 extra gallons."

 

You mean you have been dragging a bike under your car for several tankfuls and never bothered to look under your car?

 

I just had the noise develop 2 weeks after an oil change. I ripped the door off finally, as it was pretty much destroyed by the time I pulled over to see what the problem was.
I never suspected that this might be an issue with the car, but I see lots of discussion and the design seems to be inadequate.
Now I have to see if the dealer will replace it at no charge to me.....
Wish me luck, as the dealer is a greedy b*****d where I live.