Rattling at low speed when need to switch between EV and gas engine

Ignition misfires can give these symptoms. Original spark plugs? By odometer you're not quite due for replacing them, but close...

Be aware that amazon and ebay are chock full of counterfeit spark plugs. Best to buy genuine Denso brand parts from a local distributor.

 

I believe you are referring to when the engine starts up and is barely under load while the vehicle going about 40-50 mph. The engine vibrates annoyingly. It is part of the hybrid drivetrain. I had a 2013 Lexus GS450H with only 23k miles on it that did the same thing although it was MUCH less noticeable. Not long after I bought my Prius C I had a Toyota mechanic drive the car with me and he said it is a normal vibration, "Just don't drive it with the engine in that speed range" was his answer. At that time I also drove a much newer Prius C with far less miles than mine. It also had the vibration......

 

There is something drastically wrong with that story.
I've had 4 C's now and find it impossible NOT to "use the engine" in any speed range above about 5 mph.
You can NOT run a C just on the battery, unlike some of it's big brothers.
(Unless you are coasting of course.)

 

Changing your transmission fluid might help resolve the issue. I had a rumbling type noise at 35-40 mph. I changed the atf(at the shop/ drain and fill of about 3 qts) and the car runs more freely and I seem to have more zippiness in EV mode. I have a 2012 prius. I know the topic here is a prius C but thought that my info may help.

 

I too am having a grating/ rattling noise at low speeds around 30mph, while accelerating mostly. Although, it is not as extreme as the original poster Pejman described. There's no shaking that I experience while at speed. It does seem to "drive out of it" as the mph climbs. It does make sense that it is transmission related. I noticed this sound for a while now, maybe over a year ago. Seems to be more pronounced in cold weather. My 2012 prius C is at 115k miles. Hopefully a transmission fluid change will cure it. The next maintenance will be a big one. Replace engine and cabin air filters, spark plugs, transmission fluid, engine coolant (didn't get done yet) and engine valve clearance (that probably causing the engine ticking). I'm wondering what my hybrid battery health is too.