Strange noise under the bonnet after turning the engine off

Warm to hot day when I've been driving with the airconditioner on. Turn off the engine. There's a noise that's bloody hard to describe - maybe a squeal - coming from the back, passenger side of the engine bay. Back where the aircon is located so my Dad suggested something escaping under pressure from that and that would fit with the noise.



Any thoughts on what I should be looking for?



Just had a thought. Maybe it's air being forced from the coolant tank. I've just put a new cap on it but it's an aftermarket job (for the MGF). Might take it off and check it's sealing properly.

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