Engine rattling shaking sound

Hello everyone,
I have bought a Japanese toyota prius 2012 hybrid with 240000 KMs on it.
Have done the EGR cleaning along with other components.
Changing engine oil after every 4000 KMs using 0W-20.
It has no engine oil consumption i check it after every oil change and engine and inverter coolants are at their own levels so there are no leaks or consumption or anything.
The other day when I started the vehicle the engine made rattling or knocking kind of noise for around 5 seconds then it went away by it's own it has happened 6 months ago as well so this was the second time it did this in past 6 months.
Should I be worried about the engine head gasket or not?
In fast driving I get about 14km/L in slow or careful driving I get around 25km/L but under normal driving conditions dailyit's between 18km/L abd 20km/L.
By the way the service and mechanics are really cheap here for example the EGR and other parts cleaning service cost around 20 USD so the head gasket might cost me around 50USD so should I change the head gasket or it might not be a problem at all.