The burnt plastic smell on the 757 is due to contamination of engine bleed air with engine oil. At low power settings the engines produce insufficient bleed air using the low pressure bleed so additional higher pressure air is used to provide sufficient flow to maintain pressurisation.
The bleed air is at very high temperature and the heat exchangers cool the air they don't heat it.
I suspect that there is a small oil seepage somewhere in the high pressure section of one of the engines