[Ramblin Fever]

Could be ANY number of things.

Can you list other details? For example, can you hear it in the cabin; or only with the window down; does it sound like it's coming from the engine, transmission, or tires?

Is the noise only heard through heater vents maybe??

Also, you said they just checked all of your fluids, pop your hood and make certain they put your oil fill cap back on - you'd be surprized!