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I'm driving at constant speed and at about every 5 seconds I can hear a grrrr sound from the engine bay that lasts for 3 seconds and goes away. And comes back again after 5 seconds. What could be causing this? It's a faint grrr sound like that of boiling water. Even if I leave the accelerator I can hear the same sound. The sound comes even if the car is parked with engine on and no acceleration.
Car is honda civic 2009. Done 150000kms.
It sounds as though it could be one of two things. Either the fan is kicking on (the main fan) because the vehicle is up to temp and there's enough heat in the system. Secondly it could be the A/C compressor kicking on. When the A/C kicks in, the fans will kick in as well.
You need to open the bonnet (hood) of your vehicle and pinpoint where the noise is coming from. You should easily be able to tell if it is the fans which are kicking on by observing what is going on. Noise and motion will tell you right away. You can also look at the A/C compressor. If you can locate it, it is very easy to see when the clutch for it is engaged to see when the system is engaged.
If this is a different noise than you are used to, it could be one of the two (fan and/or A/C compressor) is going out. If you are unable to diagnose, you'd need to get it in front of someone to do that for you.
Scenario #1: Get your engine bay/compartment cleaned (pressurised air) by the qualified service station mechanic. If you are lucky it can be resolved by removing some obstacles between the fan wings and radiator/heatsinks; like dead rats or birds or other things like, wires or strings. Also, otherwise if the radiator fan motor bearings (one for radiator itself and other for AC condensor) are wearing out the motor would be heating up excessively and the thermal events mechanism would be stopping the fans for further damages. You could also get it inspected by manually spinning (when engine off, -ve battery port disconnected) for free rotations of fan wings. Replace if needed.
Scenerio #2: Try replacing your radiator tank cap (obviously, engine cold) with a known good one, complying the oem specifications. You can also try your luck by just flushing out radiator coolant system, probably it is jammed by some debris and check after refilling it fresh. But sometimes the water pump motor and/or thermostat mechanism is bad. Replace whatever is required, in case of hard luck situations.
Scenario #3: Check for the proper lubricant level in AC compressor. Also inspect the compressor clutch assembly for proper functioning. You can eliminate this scenario by inspection driving with AC off and relay removed, for a few drives.
Scenario #4: Check for electric circuits intermittently malfunctioning due to some contact errors and/or some other components (ICs., relays etc.) going bad. To reduce your effort, you can also use easily available OBD2 scan device for any error codes registered on your car's ECM/PCM memory.
Scenario #5: Check for any leaks by visually inspecting under your car and also inside the car cabin for heater coil leaks. Inspect the drive belts.
