Confused

It's not the fluid. Can't be the fluid.

In a Prius, the transmission fluid does not bear the load of moving the car. This is different from most every other automatic transmission on the road. That's part of why the is almost never replaced- without that heat and pressure load, the fluid doesn't deteriorate at anywhere near the rate you'd see in other cars.

So, thing 1: don't let anyone change your transmission fluid because it won't matter to your current symptoms.

Thing 2, that dealer probably ought to be shamed in public for even suggesting that as a fix.

I recall you had a problem with the boot on one of the CV joints on one of the drive axles, and it wasn't clear when the boot was ripped. It could be that the damage was already done to that joint, and you're hearing the thing loosen up under stress.

I don't have a lot of confidence in that theory, mostly because there's only so much I can work out from a written description.

But it is clear to me that you need a better mechanic for this diagnostic episode... Local friends & family can almost always make better recommendations than what you'd read on the internet.

 

Your symptoms make no sense... And because there's no warning lights/error codes you need to ignore whatever weird experience you're having until it changes or becomes worse. Trust your mechanic when he says he can't find anything wrong, which would of save you $200.

Toyota Stealerships are warranty repair specialist trained to ensure the repairs are expensive as possible because that's how they get leverage with their regular payments to Toyota. They literally could not repair or clean their way out of a paper bag without telling you that you need to buy a brand new paper bag at tens times the cost of what that bag cost elsewhere because they will claim you can't clean or repair anything, only replace with brand new.

 

Motor mounts take the blame for any kind rough engine running, strange noises etc.. and will appear physically damaged if that's the problem. Back in the day when we all drove giant 6 and 8 cylinder engines motor mounts failing were a relatively common problem after 100K miles. But these days with these small engines smooth running engines that go 300K miles without any issues if someone comes on here saying they need to replace the motor mounts they usually come back later and say that didn't fix anything and they're still trying to figure out the problem.

 

I would go back to the CV joint theory. Changing a boot is just changing the covering of the CV joint. A bad CV joint makes noise, particularly when turning.

A CV joint has to transmit all of the power from the transaxle and engine to the wheels. It is critical and you have two on each axle upfront. This is not Hybrid specific, many and most cars have CV joints. So finding a better mechanic, ideally one that specializes in front end work, would be my next stop. Most good shops have alignment machines and are better than Aamco. Never go to a franchise repair shop! Their wrench turners are not experienced mechanics.

 

Never heard of anyone needing to change a motor mount on a Prius c. Typically that happens with 15+ year old cars, cars with very powerful engines, and cars that have oil leaks- the oil degrades the rubber part of the mounts. The typical Prius c doesn't fit that at all.

Minor point to clarify- that mount would have been under the transmission and hybrid inverter, not battery. The hybrid battery is under the back seat.

This doesn't sound like a hybrid-related problem, so don't feel boxed into taking it to a hybrid mechanic. You just need a good one.

 

Your son is probably stressed out about your inability to afford repairs that are not necessary... That's not an issue with your mechancial knowledge, but your desire for a 10 year old car to be as perfect and quiet as a brand new car. Old cars make noises because they are old, not because they are broken. Call your kid up and tell him to go out and do something fun with his life on Friday and that you aren't going to worry about any of this anymore. Once the noise gets really bad in years to come get some new axles put in. But in meantime rest assured your car doesn't have any serious issues that need immediate attention.

 

A cv joint has the potential of breaking completely. If that happens you are not going anywhere without a tow.

By the way dealers often have one or two good mechanics, a few average guys and many trainees. The dealer’s service advisor usually has never been a mechanic and has to relay info from the guy who looked at it. My dealer now has some college girls as service advisors.