[speedygonzales]

So if you add a mechanical event like bumps, the sound appears. It could be something that is moving becoming disrupted by the event like a motor as you stated. Or something that is not moving becoming disrupted by the event.

Assuming it's something moving like the blower motor, be sure it's off and test it with a bumpy road. If need be, pull fuses to turn things off that don't have it's own switch.

One other thing you can try is tapping on things with a rubber mallet. The only thing that really makes sense is the blower motor which is by the passenger air bag. You can easily tap on it. Our blower motors are the worst. Always noisy.

I have heard relays oscillate before. Just a thought. Unplug them and try the same event.