The heater matrix in a car’s engine is the source of many of the sounds you might be able to hear coming from under the bonnet. A lot of the time, you will have to take the dashboard off to access the matrix, so you may wish to pinpoint the exact source of the sound by asking a professional. If the issue is the heater matrix, you should ensure you resolve it as quickly as possible or run the risk of the car incurring significant damage over time.
Hissing from heater matrix
If it sounds like you’ve got a snake living under your bonnet and you don’t live in a place where that’s even a remote possibility, you’ve probably sprung a leak in one of the hoses that connects the heater matrix to the rest of the engine. Whether this has occurred at the connection point (which may mean that a clamp has come loose) or elsewhere along the hose (which probably means that a small hole has developed through which coolant and/or steam will be escaping), it’s a problem that will need rectifying as soon as possible to prevent significant long-term damage occurring to the engine.