Car knocks When Changing Mg2 to ICE in a hill

I recently ( 1 month back)changed 12v battery (after 5 years) of my 2010 3rd gen prius.and recently changed the engine airfilter. when i drive in a hill (40-45 degree slope) sometimes engine knocks and misses for a while with a heavy noise.this happens when its change automatically from electric motor to the engine. During starting of the ICE it knocks ,this happens sometimes.not always and this happens only in a hill.not always.i noticed it on display bar when ICE kickstarts it knocks.in a flat road no issues at all.fuel consumption average 23-25km per litre.egr cleaning done 30000km back.want to know when running on electric motor(not ev mode) when battery gets down to low displays 2 bars then automatically its kickstarting ICE,from where it draws power to the mg2 to rotate engine?during that first rotation of engine it knocking vibrating.it happens only in a hill(climbing 40-45 slope), the thing is, it not happen everytime once in a 100 time

 

Thanks! Is it regarding engine oil issue?or engine oil pressure

 

Last week changed the engine oil after 9000km,using only 0w30 mobil1 full sunthetic oil.oil changed at regular intervals.once i used castrol edge professional 0w30 full synthetic that few years ago.then again switched to the mobil1 full synthetic.always used toyota filters.oil level is ok.no burning issue can u please explain

 

Hybrid repair center here says the issue is may be fault in battery pack

 

135000 km

 

Transmission fluid issue? This engine knock with heavy metal strucking noise comes only for a second in about 45 degree slope.at this stage car engine stalling.need some expert opinion

 

Yes.from 2010

 

Yes rufely 40-45 degrees.

 

Yeah less than quarter of liter for 9000km

 

Thanks!
Cant understand Why it happens only on a slope of a 35-40 degree climbing.can it be regarding engine oil circulation or a transmission oil level-circulation problem?

 

Thanks!
Cant understand Why it happens only on a slope of a 35-40 degree climbing.can it be regarding engine oil circulation or a transmission oil level-circulation problem?

Not exactly 40, may be 30-40 degree today it knocked in a car park with 30 degree angle