Depends on what you mean by "first start". Within a quarter-second
of hitting the button? Within ten seconds? A whole
bunch of stuff happens when you fire the system up, so
it could be almost anything.
I'll hazard a guess that you mean something concerning the engine,
and point out that while it's in warmup it's running very retarded to let
more heat out to warm the catalytics. This tends to make the
exhaust note sound a little more "metallic" or rattly. There is also some
noise from the inverters and electric motors along with that,
as some of the energy from that warmup state may also go to
charge the HV battery. So that creates a whole wash of high-
pitched harmonics for a while, which eventually settles down
as things warm up and begin changing state.
Alternatively, the "grinding" might be the coolant pump, bring in warm
coolant from the thermos bottle before lighting the ICE. That
pump tends to be a bit noisy.
Can you describe it any better and provide a more exact timeframe?
When you first turn on the car, you get a few clicks in the rear of the car as the system relays engage.
Also during this time, depending on engine coolant temp compared to coolant heat storage container, a servo valve may change position, after which the coolant heat storage pump runs.
When you press on the brake which is required to put the car into READY, you most often engage the brake accumulator pump. I believe this is the sound you refer to. Sounds like a loud aquarium air pump.