Got a weird one here.
I'm hearing an intermittent "hoot" sound that is higher in pitch than an owl would make. Probably in the ballpark in frequency of the tone that's played with color bars on a TV set/station test pattern (1kHz). If I had to guess, it's probably aroudn 700Hz. It doesn't last long, maybe a second, at most I've heard it for 4 seconds continuously. Crops up once every few minutes at most, seemingly randomly.
I've heard it happened while: stopped and idling, coasting at low speed, starting off and going up a hill and ... here's the one that gets me: even JUST after the car has been turned OFF.
Seems more frequent on hotter days.
Doesn't seem to vary with wheel speed or engine speed.
I've not yet heard it when the Air conditioning is off (but I just tested this on 2 drives yesterday after the idea came to me). The A/C system was serviced about a month ago. So my best guess is the air conditioning system somehow.
What in the A/C system might cause such a noise I wonder? Or how could someone addressing a cooling system leak have screwed up to cause this to happen?
Or could it be something else still? Alas, I could not find an owl in the engine compartment to explain it otherwise.