I have had similar noises it was the A/C system. When I have turned it off, then the sound stopped. Took it to A/C specialist then for like 40 GBP, A/C cleaning, refill etc. and it stopped.

1 more thing. I have had a head gasket blown in my off-topic car.
The temp NEVER went above half. Thermostat was new, everything seemed normal, except a few drops of coolant from the overflow bottle. The end of the story was that later it started to "burble" the water by pushing the air into the water chamber (not sucking it or whatever). Solution was head gasket repair...

Hope these help...

Cheers,

R.