A.O.A
Dear Hamad,
when we start the car in morning the engine starts on petrol after few seconds to heat up the engine, and after approx. 20~30 seconds it stops and the engine is converted to the operate through battery. Also if we stay in heavy traffic the battery drains and engine starts on Petrol to charge the battery, once it is sufficient charged the engine stop using the petrol. these are normal conditions. the sound is of petrol engine which starts and stops, you also have experienced the sound of engine while you accelerate but that due to movement of car sound is less heard.
if the engine sound is extra ordinary high do get is checked with some local hybrid mechanic.
I reached Toyota for my Aqua and they refused to do any work and said we do not produce nor Import this car so cannot service or provide any support for such cars.
in addition other group members has suggested oil and filters go for it.
hope your query is now clear.
Thanks.