You can get a growling noise after a vehicle is turned off from the cooling
system.  Take a look at the overflow/coolant bottle just after the vehicle
is shut down, If you see bubbles and noises you have a head gasket gone or
starting to go. You can have a bad head gasket without overheating.

I have a 1995 Nissan Pickup SE with around 114,000 miles.  Within the
last 6 months I've noticed a vibration /growling noise coming from the
truck when it's NOT running and the ignition is off.  I thought at
first the noise may be coming from the emergency brake, however I've
tested with the brake off and on a level surface and the noise still
occurs. The noise only last few a few seconds and  seems to happen at
random times. I never hear it when the truck is running.  Any ideas
what this may be coming from?  I'm baffled.

Thanks!
Dave

It could be the antenna.  Mine does this sometimes because it is
broken and the motor is trying to retract it when the vehicle is shut
off.

Paul

My gas tank cap baffled me for some time.