Strange sound when press clutch

Hello. It has been a while I notice on my BMW E87 2006 that from idle engine when press the Clutch to put in 1st gear usually I hear a strange sound from below the car. It sounds like transmition or differential gears even without put in any gear yet or sometimes like exhaust gasses and this is in different places at length of the vehicle. Though this is a one-time sound but happens almost half of the idle times on the road. I also confirmed the same issue exists on my wifes car Toyota Auris 2010. What is this? Sometimes it sounds pretty loud. Is it normal?