What is the sucking sound coming from the engine when you switch it off?

Answer 1

I'll leave the other comments below, but as a Toyota echo owner, I bet the sound you refer to is the sound of the fuel tank equalizing pressure, I own two echos and both do this, its a very odd "feature" of the echo, mine sound like croaking frogs, it can go on for quite a few minutes after you park the car. I assure you this is perfectly normal and there is nothing to fix. If this is the sound, you should be able to hear it much louder when you put your ear next to the fuel inlet.

The sucking sound is going to be already there but you can more than likely only here it for a couple of seconds after you turn the engine off.

This is more than likely going to be a vacuum hose or pump of some sort and usually depending on the type of vehicle you have I would take a guess and say the thing that you would usually expect this type of noise to be coming from the Brake Servo Vacuum Hose (Pressure Releasing from the unit) this maybe the result of having an air leak in one of the hoses and therefore creating this noise once you switch off the engine. In Normal circumstances you would not hear this because every hose and the unit itself is contained and sealed therefore releasing the pressure over a period of hours not seconds.

It certainly points towards an air leak of sorts even if you find it is not the Brake Servo. There maybe several other vacuum hoses located on the engine so it is just a process of elimination. I would definitely check the Brake Servo first though. just by placing your head next to the unit (either Bulk Head left or Bulk Head right) which is usually a black unit with the Master Cylinder (Where you put the Brake Fluid in) mounted on the front of it.

Hope this Helps