Whistling sound from engine

[bullsta]

Forgive my ignorance on this one.
Today I unplugged my MAF harness plug on my GTI to see if the MAF needed cleaning.
Idle stayed the same when plug was removed, but when I plugged the MAF plug back in the car immediately stopped.
I turned the ignition off and then restarted the car with MAF plug still in and the car started and idled fine but there is now a barely audible high pitch alarm sound coming from the engine which stops after about 20 seconds and then a whistling sound (air escaping) coming from a round plug with a small pin hole that sits just in front of the coolant reservoir.
Weird as I have never heard these sounds before and I know the car well.
Any ideas?

[h100vw]

Forgive my ignorance on this one.
Today I unplugged my MAF harness plug on my GTI to see if the MAF needed cleaning.
Idle stayed the same when plug was removed, but when I plugged the MAF plug back in the car immediately stopped.
I turned the ignition off and then restarted the car with MAF plug still in and the car started and idled fine but there is now a barely audible high pitch alarm sound coming from the engine which stops after about 20 seconds and then a whistling sound (air escaping) coming from a round plug with a small pin hole that sits just in front of the coolant reservoir.
Weird as I have never heard these sounds before and I know the car well.
Any ideas?

First lesson from that incident is about unplugging things with the ignition on. Not the done thing really. I am of the opinion that MAFs work or not.

"Flamesuit on" cleaning one is a pointless exercise, swapping in a known good one is the only way to prove a bad MAF.

Also noteworthy, if you disconnect one and turn the ignition on, it will log that in the ECU. So you need a way of clearing codes. If you have a way, VCDS maybe, then you can see the output of the MAF anyway and log it to prove its ok.

Your second one is the EVAP canister and at idle the ECU will trigger the solenoid you can hear and allow tank fumes to be ingested by the engine.

Touch the solenoid while you hear the whine you'll feel it buzzing.

Gavin