If that's the case, then I'd take it back to the dealer and leave it with them a few days when cold weather is expected. Since your paying in the neighborhood of .55 cents a mile for the first 36,000 miles to own it, let them earn it.
Is the car new or how many km's/miles has it done? Is the grunt a sort of a buzzing sound? Could be damage in the pump or the pump is drawing in air from somewhere.
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