2014 Prius v, 198K, vibration in drivetrain?

This car has been a tank... no problems at all since buying it new, until I took it in for maintenance last month and had my first non-maintenance repair ever... dum da da dummmmmm... two rear license plate lights.

Pushing 200K now, and there's this weird vibration that I've been feeling occasionally. It happens at low- to mid-throttle acceleration at low speed and most observably when the hybrid battery gauge is low. It feels like engaging first gear from a standing start in a vehicle with a manual transmission that has a warped clutch plate. Sometimes I will hear a rattling sound, appearing to come from under the hood, that matches the vibrations. I can also feel vibration when I'm at pretty much any speed and low throttle (such as if I'm on cruise control), though it's most noticeable at highway speed. (At these times it feels like something in the power split device can't decide if it wants to be engaged or disengaged.) I notice this mostly when the engine is already warmed up, and it is noticeable when the hybrid battery shows "normal full" (6 bars) though not as much as when it's low.

The engine idles fine, runs smoothly under all conditions with no feeling of "missing", there is no vibration at speed if I'm coasting, and I can get the vibration to stop by stepping the accelerator down harder and getting more vigorous acceleration. No "check engine" or dashboard lights of any kind. I just got a new set of tires and rear brakes, and had the front brakes adjusted, so they can be ruled out as suspects. Toyota dealership multi-point inspection revealed nothing, though I didn't mention this issue for fear of severe financial agony. It legitimately feels like there's a warped clutch somewhere in the power split device. Anyone else had this issue? If yes, what had to be done to fix the problem?

 

Oil was changed less than 2,000 miles ago. I always ask about the amount of oil that comes out and what it looks like, and nobody has ever said that I was low on oil or that it was alarmingly dirty. I have kept up on the maintenance all along.

"EGR circuit cleanse" - nope, no clue about that. I was reading some of the other threads on Prius Chat about engine vibrations and the like, and some mentioned cleaning the EGR circuit, but their engine problems didn't seem to match mine. They were talking about the engine feeling like it needed a tune-up or injectors, and idling rough, and whatever. My engine runs like it's brand new. I've had engines that needed a tune-up before and it's most noticeable under hard throttle acceleration.

I suppose I could look into getting the EGR circuit cleaned out... any idea how much that's likely to cost?

 

I have noticed that the HV battery is acting differently from how it used to act. The gauge seems to drop faster than it used to under the same load conditions, the engine kicks on and spins up as I charge the battery, before the battery is full (at 6 bars on the gauge instead of the full 8), and sometimes it seems like it takes an unusually long time for the battery to charge up after I've used it for a bit. Sometimes, after the car has been on EV mode for a little while, the engine will inexplicably kick on at a red light even if the HV battery isn't full, the heat is off, and the engine is warm. My gas mileage has been low lately but I've attributed that to using winter gas. I've always seen a noticeable drop in fuel economy when they switch to winter gas.

I don't know if this next bit of info will help but I can say that I have run the battery through a lot of load cycles. I'm kind of a nerd and I've figured out how I can stay in EV mode as long as possible so that I can squeeze out a couple more MPGs over what I'd get ordinarily. (To put it in perspective, my wife drives the Prius "normally", and her MPG numbers always lag mine by a significant margin.) I'll use the cruise control to slow down instead of the brakes, which will recharge the battery. I'll try to drive at 43 mph or lower (the cutoff point for EV mode when you're accelerating), I've recognized that at 46.5 mph and lower I can let off the gas quickly and get it to drop into EV mode even though it wouldn't do EV mode beyond 43 if I were accelerating, etc. Simply put, I'm sure this battery has gone through quite a few load cycles on my watch, though most of our driving is highway driving. (We got 99K miles on the OEM brakes and though we had to get the rotors and rear pads done again after another 97K, the front pads were "almost new" after that same 97K.)

Not sure I could produce any more relevant info. Back in the day, even when the HV battery meter was at its lowest (2 bars), I could tell that the engine was providing most of the accelerative power, the acceleration was not as sprightly, and the instantaneous gas mileage was quite low, but there was never any vibration.

Even if what you're saying is true, why wouldn't the gas engine be able to power the car without vibration even if the power it produces is not as much as it can produce along with a new HV battery?

And as for that "Dr. Prius" app, my wife has a relatively new Motorola (Android) smartphone. Can this app be found to work with this phone? If yes, would I need to buy any fancy equipment to work with it (such as a plug-in unit for the OBD II port that can transmit a wireless signal to the phone?)