Wheel bearings will cause a rumbling noise consistent with the revolution of the tires. Your highlander is doing a computerized check of systems. If your noise is like mine, when you start the engine and pull away you hear this noise and then it goes away.
the spare tyre.
Transmission problems.
Bad CV joint, loose rim, Loose caliper
A bad tire or a bad wheel bearing.
Check the drive axle for a broken and leaking boot or bad CV joint.
As the moving car approaches the person in front of it, the pitch of the car horn will sound higher due to the Doppler effect. This occurs because the sound waves get compressed as the car moves closer, causing the frequency to increase and the pitch to sound higher.
Something loose or broken. Try to determine if its when going over certain bumps or down straight smooth terrain.
its the sound of the pistons stroking.
that's just the sound of the steel bits cooling down
A vibration at 20 Hz would be a low-frequency sound that is felt more than heard. It may produce a deep rumbling or pulsating sensation, similar to the feeling of a subwoofer in a car audio system.
electrical interference
You forgot to let your wife in